YOU is modified by various commands such as:
c_scan, c_trace and so on.

If we take c_trace for example you points towards the entity hit by the trace-beam.

You can also use the YOU pointer to ignore certain entities.
Lets say you have a non-passable gun as level entity and are always reseting it to the players position.
It would certainly block the player when he performs any _move instruction (c_move, or ent_move)
So if you have a pointer to the gun "activ_gun" e.g. you
can store it into YOU and ignore it:
you = activ_gun;
c_move(my,move_dist,nullvector,ignore_you);

There are lots of possibilies for the use of YOU.