it advances the timing, equivalent to turning the distributor on an older engine.
and doing so it makes more power. do this on NA engines ONLY, anything with forced induction should not advance timing.
you dont want to advance much if any on a daily driver IMO.
Well a big part of tuners is advancing the timing. It does help. The Mopar PCM in my truck made a huge difference (it will ping on demand with 87 octane, so it added a lot of timing), the tradeoff is having to run premium but it gets the same mileage so it's not much more money.