Well, back when Battletech started Assault Mechs carried devastating close range weaponry and the armor to get them into range. Generally an AC/20, 2 medium lasers, and an SRM-4/SRM-6 (Victor and Atlas carried this arrangement). Their job was to assault the enemy position while under supporting fire from mechs like the Catapult or Warhammer and force the enemy to displace from said location or get hit with the close range death dealing weapons.
However, Battletech changed, so that now the death dealing weapons are the long range ones rather than short range ones, and thus we have the game we play now, where Gauss Rifles, ERPPCs, Large Lasers, and LRM-20s do all the work, and weapons like AC/20s and SRMs are reserved for one-trick pony flanking attacks on fast mechs, defensive duty against such attacks, or desperate charges against foes camping outside your hangar to try and push them back or draw fire long enough for your allies to pick off the enemy's heavy hitters.