Why do they have numbers on top of buses?

Question

Why are the numbers of a bus on its roof where no one can see them?

Answer 1

helicopters can see them, if one gets jacked or something they can find it easily from the sky

Answer 2

But they can see them.  

Supervisors and law enforcement can see the numbers and know which bus they are looking at in the event of hijack or other problem,   or simply to moniter the safety level of the drivers.     Helicopters,  light planes,  and tall buildings can be used for these purposes.

Sidenote --  many trucking companies do this also to their trailers to assist law enforcement in locating stolen equipment.

Answer 3

There's actually a much less exciting answer.  When the buses are parked in the yard, they're parked in tight groups and the only way to know the number of the bus you're seeking is to climb into the lot tower and look for it.  This lets the workers know what bus is where on the lot and how to most easily get it.

Being able to track the bus in the air is an added benefit, but let's be honest, how long would it take a ground based police force to find a rogue bus?