Aftermarket high end trigger sets do the following:
1. Increase Durability
- they are usually made out of steel, brass (bushings) and precision cut parts
- high powered spring guns put a lot of stress on the trigger set...and stock pot metal just can't hold up to it
2. Improve feel
- the machined angles and precision surfaces typically result in smooth, predictable trigger pull
- imagine dragging a bag on the sidewalk...rough and jumpy. Now polish that concrete until it's mirror smooth...there's a lot less drag and an nice smooth pull
- some stock triggers are adjustable...others are not. High end triggers allow for precise, fine tuning of your trigger pull.
Mostly...they're for durability. Pop a 450fps spring into your gspec and start cranking off shots...the stock TM sear won't last long before it's basically sheared off by the force of the piston being pushed forward.
"Target" triggers are often built with the safety omitted...not a big deal for target shooting. Not having a safety on a "field" rifle is stupid.