What are the roles that are used for these guns in airsoft?

Sniper is pretty obvious i would assume. Longer range shooting. however most airsofters in my area at least are starting to use DMRs more for a quicker second shot.

Machine Gun is for support. Helping your team move from cover to cover. I had a M249 for awhile and loved it. had it at almost 38rps. kept heads down nicely.

Assault Rifles are what you would expect. somewhat close to mid range shooting. Unless you convert one into a DMR then it would be long range.

Sub Machine guns are used for CQB mainly because of the maneuverability.

Shotguns and pistols can be grouped together. Some people use them both for CQB and mid range shooting. Both are normally used as backups from my experience except in some CQB environments.

Sniper Rifle = Ranged Shots (Getting those that are too far for other units to shoot)

Machine Gun = Support Fire (suppressive fire)

Assault Rifle = Frontier (Pushes Forward)

Sub-Machine Gun = Scout (more mobile than assault, usually more accurate, sometimes not as accurate as pistol depending on barrel size)

Shotgun = Frontier / CQB (depending on what you get, if you have a single shot, it's not practical, if you have a tri-shot / shell ejector, you'll get a nice spread of bbs)

Pistol = Sidearm / CQB