Critical seeking is an important BATTLETECH tactic to understand and master- especially when going against heavy and assault mechs.
When you finally break through to the internal structure of a mech and cause damage, there is a chance of causing critical component damage, followed by a roll to see what was damaged.
Much depends on the location, but heat sinks, engines, sensors, weapons, and ammunition, which can explode are all possibilities.
Critical damage is a way to not have to chew through all the armor of a mech.
Crit-seeking is where as a mechwarrior you look to actively breach armor and cause critical damage- which is done this way.
Once internals are exposed, the next hit- be it a PPC or a machine gun has an equal chance of causing a critical- firing many smaller weapons or short range cluster weapons- machine guns, small/medium lasers, SRM packs increases the chance of hitting that crit spot.
Taking mechs that have a variety of weapons like this- the Battlemaster being a great example.
The PPC to punch trough armor, the medium lasers, machine guns, and SRM pack to crit seek.
The Warhammer is another great crit seeking mech.