Role is holistic, a weapon does not define a role.
What is the best use for a unit? How do you use it best?
Take a BattleMaster and a Panther.
Both have a PPC. If you take a lance of Panthers versus a lance of BattleMasters, and they sit at long range and snipe away, the BattleMasters will win. Is that because you used the BattleMaster correctly? No, it’s because a BattleMaster versus a Panther isn’t a fair fight.
Take a BV or PV equivalent force of Panthers versus BattleMasters, and the BattleMasters sit at long range sniping and they very likely lose. They pod a lot for short range weapons they aren’t using. But run them up to short range and they will are far more likely to win.
Take an Atlas and an Imp. Both are tough Assault mechs. How do you best an Atlas with an Imp? Stay at long range. How do you beat an Imp with an Atlas, charge up close.
An Atlas can fire its LRM-20 at long range. And maybe due to other considerations with your force, you want to do that. But it’s not because it’s the best use of the Atlas.
Roles are often down to two things, speed and range. Answering those two questions almost always answers what role.
Slow and short range, Juggernaut (or Ambusher if low armor).
Slow and long range, Sniper. If mostly indirect, then Missile Boat.
Fast and low armor, must move fast to survive, striker.
Fast, but tougher. Skirmisher.
Fast but weak, “Worthless piece of junk”, Scout. (Not fond of that role btw).
Fast and long range, gotta decide which is more important, the speed or the range.
Brawler, more complicated. Has to have some speed (4 walk), and some range. But isn’t dedicated to range or speed. The idea being they are the line. They can’t be forced to break the line because they have no range, but they have to be able to stay up front even if charged and not be overwhelmed. The BattleMaster is toeing the line between Brawler and Juggernaut. For a SW era mech, I tend to be more lenient and let it be a Brawler.