About armor:
The armor is protecting the body. If you tear off the armor you can then shoot at the unprotected body.
It's like every other RPG, except in this game there is actual armor that you can remove to get at the vulnerable part and not a percentage statistics you debuff.
Knocking parts off does the following:
1) You can use the parts as materials
2) Some robots have weapons that can be used when knocked off
3) Knocking parts off lowers their defended, in some cases it causes an explosive damage
4) Last and the most important reason, knocking off a certain part mean the robots lose a certain ability.
The way to quickly kill a robot is by doing the following in this order:
1) Lay traps before fight
2) Knock out the important stuff using tear arrows, it might do some damage it might not....But damage isn't important here.
3) Debuff the robot using an elemental weakness
4) Use heavy arrows at critical points, if possible fire 3 arrows at once provided you have the skill.