Want to avoid your beer slopping all over the surface of your table until it forms a monolayer that inevitably seeks to escape the tabletop by dripping off the edge of the table that is directly above your thigh?
Use beer mats!
Want to demonstrate that you've been out drinking?
Collect beer mats! (This is particularly effective if you are too young to go out legally drinking, and want to impress your fellow under-age drinker wannabes. Of course, the age at which you stop being underage varies enormously from place to place.
Student unions are too poor to have beer mats. Go to actual pubs.
US & Arab-country specific hint: Especially avoid the students unions that do not provide bars, sticking rigidly instead to cafeterias and restaurants.
UK specific hint: Remember, it's legal to be taken to a pub accompanied by an adult, even before you reach drinking age. While you sit there, sipping lemonade or Irn-Bru (made sophisticated by the addition of ice cubes and the occasional slice of lemon), feel free to acquire some beer mats that you can then show to your mates as proof of your hardened drinking habits.