Going deep gives you one big advantage: reserves. Even if your front line gets broken, there is another line of infantry step up and stabilize the situation. If you have only one line, then, generally, once that line gets broken, the whole army routs. Also, your 2nd and third line troops are rested, and if need be, they can be peeled off and respond to developments on the battlefield, like at Pharsalus.
Romans used triplex acies for centuries. Clearly it had something going for it. Here's what Wikipedia had to say about it: Roman infantry tactics - Wikipedia
Right, but again, why have them pressed right up against the back of another Unit when you can have them farther back so they can both reinforce whatever area of the line is breaking (not just the area they are pressed against), while also preventing total envelopment?
Oops, I accidentally responded to you twice, I meant to respond to someone else and it doesn't let you delete replies.