Increasing timing increases the RPM of a motor and reduces Torque. The motor will still make the same total horsepower, but its powerband will be different. Its a trade off.
Use more timing on tracks with lots of turns, jumps and short straights with a smaller pinion gear. Higher timing also drains the batteries faster, and causes more arcing in the motor so use a slightly stiffer brush spring.
Use less timing on larger, tracks with more straights with larger pinion gears.