- increase fuel economy
- tune car for best performance
- add power mods to your car (nitrous, supercharger, transmission, etc)
- eliminate knock or other engine problems
Doing anything to your car yourself is intimidating if you've never done it before but mechanics often try to rip you off and performance shops will charge you for stuff you can do for free with little effort.
Doing certain types of work to your vehicle does indeed require training and experience but more often than you'd expect, if you want to add 10HP to your car or pick up an extra 3 MPG or get rid of an engine light, you just need to KNOW what to do and then you'll realize that was the hardest part because the tools and skills you need to achieve these kinds of results can sometimes be next to nothing.
Tell me what kind of car you have and what your goal is and I'll let you know how to make it a reality.