Two things:
1) Gauging distance to tell how far away the opponent is. That information is crucial to hitting them, and preventing getting hit by them.
2) Threatening the opponent. If you do want a specific distance, the opponent must have a deterrent to advancing. Think of a knife fight. If you have a knife, better to threaten your opponent with it if they also have a knife, as opposed to keeping it closer to yourself and less of a danger to them.
Examples:
Some people would attribute Rigo's ability to NOT use his right hand as defense to his speed, but that's actually backwards. He uses the technique of consistently poking and prodding with his right hand to ascertain how far the opponent is, make his attack, and recover to a safe position. It just makes him look fast when the opponent's reaction is always a step behind.
Many people like to think Golovkin's opponents are there to be hit out of fear. That they fear him so much they just don't do much when he fires. But the fact is they're not sure what range he's at while he's sure what range they are at (mostly, when he's unsure, you see him get hit back, he just doesn't always care). He essentially uses his left hand to say "stay right there"...then throws his power shots.
Erickson Lubin fights very similarly:
These guys just trade the perceived risk of less defense for control of distance, which ion Rigo's case, leads to easier acquiring of angles. He makes a move at a certain distance, the opponent sees it and tries to adjust and attack, but is too far away to actually hit him.