If you've shaven the head, and have aftermarket cams, the gears will help dial in the "recommended" timings provided by the manufacture. Even on an untouched head, some cams may be off a few degrees from mfg. specs. Cam gears help dial them back in to specs.
Aside from that, adjusting them beyond specs can improve or decrease performance, but most likely, you shift your power band around. This topic has been beaten to death, search for more specifics.
hopefully you dont mill your head enough to affect cam timing, a few thousandths isnt gonna throw anything off, however if you are looking to squeeze out that last little bit of power, you can play with the timing of the cams and usually either increase torque or horsepower depending on how you set them up, but in order to make them work properly you will need to degree the cams and decide what you are trying to increase.