Boot up any computer (regardless of OS) using a live system (eg. Ubuntu Desktop or a flavor and use the TRY mode) and explore the data on the disk... Without encryption you ancan quickly get access to data etc (any OS, from CP/M from the late 70s, DOS of 80s, or any modern OS too), and even change that data! (eg. if you forget a password; replace the hashed password with a known hashed password; same with Microsoft Windows or a Ubuntu) ...
Encryption makes that immensely more difficult, and puts out out the range of possibility for most end-users anyway.
There are various methods of encryption too, some make this more difficult than others, some encryption methods require specific hardware too; so whatever Ubuntu product/release you're using will have specific ways it'll encrypt, which may differ to options used on a different Ubuntu product/release and with different hardware.
If you have good physical security, encryption just adds more complication to the maintenance in the future usually (ie. desktop/servers in offices/homes with few visitors etc*), but if the device is regularly transported (laptop, tablet etc), the added cost of maintenance is easily justified with the additional security offered by encryption in my opinion.