They were initially provided just to give new players some idea of the sort of function a 'Mech was usually going to have in a game.
So for example, "sniper" doesn't mean "sits on a hill 5 miles away with a telescopic sight", it means "tries to stand off at longer ranges and use direct-fire weapons". The general intent that goes with this is "I should be firing at my medium ranges when he's firing back at me at long ranges" sort of thing.
"Missile boat" - similar, but with missiles rather than direct-fire weapons. Hiding behind a hill and using the services of a good spotter also applies.
Now the Atlas has an LRM-20, but isn't a missile boat. Its weapons loadout is about getting in your face, while its size & armour help it get there. Hence it's usually considered a Juggernaut. Brawlers are the lighter cousins of Juggernauts - they work best up close, but need more help (or luck) to get there & survive there.
And so on. People who've been in the game for a long time have no problem deriving this from the stats, but the newer players can find this useful.
When Alpha Strike came along, they decided to use these to add some flavour. A formation - eg. a lance - of 'Mechs which all work the same sort of way will do better than a mixture. So if your formation consists 3/4 of Missile Boats or Snipers, then it gets certain benefits. If they're mainly Missile Boats with indirect fire capabilty (MRMs need not apply), then you could have a different benefit, & so on.
So it's just a flavour thing there, but one I and other people happen to enjoy playing with.
Helps?
W.