We get this question very often.
Bearings in roller skates are for the wheels to move freely and easily on the skates. The higher-rated the bearings, the better. You will need 2 bearings for each wheel, meaning quad skates will have 16 bearings in total, and the same goes to inline skates with 4 wheels.
Remember to clean your bearings each time you go roller skating to keep them under good maintenance. This, however, can be very time-consuming.