So far all the answers address the purpose of a round-trip ticket to the traveller. But it's the airline that makes the decision to offer round-trip tickets for less than the price of two one-ways, if they do this. So what's in it for the airline? I can think of three possibilities:

• Customer convenience: if travellers see the process of booking an airline ticket as a pain, being able to book both legs of the trip in one operation may be a more attractive option. Perhaps this is a hangover from the days of travel agents and paper tickets which is less applicable nowadays.
• Fixed and variable costs: if the booking process costs the airline more than a negligible amount, they may be able to save some of this cost when a customer makes both bookings in one transaction, and can therefore pass on some of this saving to the customer.
• Tradeoff of cost and convenience: If all prices are one-way and airline X has the outward flight I want to take but airline Y happens to fly at a more convenient time than X on the way back, I'll fly out with X and back with Y. In that case it's in X's interest to offer me a discount on their return flight, so they make a reduced profit from me on that flight rather than none at all.
• Market segmentation: It's quite likely that travellers who want to book one-way journeys are different from travellers who are willing to book a round trip. Maybe they're more likely to be flying on business or at short notice. In that case they may be willing to pay more, on average, for their flight than round-trip travellers. So what appears to you, the round-trip customer, to be a discount is in fact a premium being charged to one-way customers because that's what the market will support.

If I had to bet which of these was most significant, it'd be the last one.

Many of the answers address the purpose of a round-trip ticket to the traveller. But it's the airline that makes the decision to offer round-trip tickets for less than the price of two one-ways, if they do this. So what's in it for the airline? I can think of four possibilities:

• Customer convenience: if travellers see the process of booking an airline ticket as a pain, being able to book both legs of the trip in one operation may be a more attractive option. Perhaps this is a hangover from the days of travel agents and paper tickets which is less applicable nowadays.
• Fixed and variable costs: if the booking process costs the airline more than a negligible amount, they may be able to save some of this cost when a customer makes both bookings in one transaction, and can therefore pass on some of this saving to the customer.
• Tradeoff of cost and convenience: If all prices are one-way and airline X has the outward flight I want to take but airline Y happens to fly at a more convenient time than X on the way back, I'll fly out with X and back with Y. In that case it's in X's interest to offer me a discount on their return flight, so they make a reduced profit from me on that flight rather than none at all.
• Market segmentation: It's quite likely that travellers who want to book one-way journeys are different from travellers who are willing to book a round trip. Maybe they're more likely to be flying on business or at short notice. In that case they may be willing to pay more, on average, for their flight than round-trip travellers. So what appears to you, the round-trip customer, to be a discount is in fact a premium being charged to one-way customers because that's what the market will support.

If I had to bet which of these was most significant, it'd be the last one.

So far all the answers address the purpose of a round-trip ticket to the traveller. But it's the airline that makes the decision to offer round-trip tickets for less than the price of two one-ways, if they do this. So what's in it for the airline? I can think of three possibilities:

• Customer convenience: if travellers see the process of booking an airline ticket as a pain, being able to book both legs of the trip in one operation may be a more attractive option. Perhaps this is a hangover from the days of travel agents and paper tickets which is less applicable nowadays.
• Fixed and variable costs: if the booking process costs the airline more than a negligible amount, they may be able to save some of this cost when a customer makes both bookings in one transaction, and can therefore pass on some of this saving to the customer.
• Market segmentation: It's quite likely that travellers who want to book one-way journeys are different from travellers who are willing to book a round trip. Maybe they're more likely to be flying on business or at short notice. In that case they may be willing to pay more, on average, for their flight than round-trip travellers. So what appears to you, the round-trip customer, to be a discount is in fact a premium being charged to one-way customers because that's what the market will support.

If I had to bet which of these was most significant, it'd be the last one.

So far all the answers address the purpose of a round-trip ticket to the traveller. But it's the airline that makes the decision to offer round-trip tickets for less than the price of two one-ways, if they do this. So what's in it for the airline? I can think of three possibilities:

• Customer convenience: if travellers see the process of booking an airline ticket as a pain, being able to book both legs of the trip in one operation may be a more attractive option. Perhaps this is a hangover from the days of travel agents and paper tickets which is less applicable nowadays.
• Fixed and variable costs: if the booking process costs the airline more than a negligible amount, they may be able to save some of this cost when a customer makes both bookings in one transaction, and can therefore pass on some of this saving to the customer.
• Tradeoff of cost and convenience: If all prices are one-way and airline X has the outward flight I want to take but airline Y happens to fly at a more convenient time than X on the way back, I'll fly out with X and back with Y. In that case it's in X's interest to offer me a discount on their return flight, so they make a reduced profit from me on that flight rather than none at all.
• Market segmentation: It's quite likely that travellers who want to book one-way journeys are different from travellers who are willing to book a round trip. Maybe they're more likely to be flying on business or at short notice. In that case they may be willing to pay more, on average, for their flight than round-trip travellers. So what appears to you, the round-trip customer, to be a discount is in fact a premium being charged to one-way customers because that's what the market will support.

If I had to bet which of these was most significant, it'd be the last one.