Why do we give ourselves pointless rules?

Would a train pass through a red signal?

Would an aircraft land on a motorway?

Would you throw rubbish out of a window?

Why not?

Control. Plain and simple.

Our entire society is built around maintaining and regulating the masses often through seemingly pointless rules and regulations. It pretty much boils down to having to maintain some sort of standards, otherwise all other standards are also ignored. Sure, eating in class won't end the world, but it can disturb others and add distractions to yourself and those around you. If everyone is now eating there's even more noises, smells, and visual distraction going on.

All these rules are trying to prevent chaos and things from escalating to disorder even if in trivial ways such as dress codes.

They are not pointless. Convention and self-discipline still have a point, even if they are meaningless to you personally.