Same opportunity to try Linux as with the LiveCD - but with the difference that the data on the USB stick can be changed, ie there can be written to and stored on the USB stick. So, besides that you can try linux operating system, you can then also use it right and save the stuff you're doing on the USB stick. So, besides the possibility to test an operating system, you get a "portable" oprerating system that can be taken into the pocket and used on multiple computers.
Note: On some computers, especially older ones, you can not boot from a USB stick - ie it can not be used. Here one must use a LiveCD instead.