Of course you also change the effective length of the main spring , by moving that lever, so it will result in an faster or slower running watch, but you will very likely see the watch running less stable.
You use this lever normally, to ensure that when the balance wheel is not moving, the pallet stone is in one line with the center of the anchor and the center of the balance.
In trivia: it regulates the difference between the "tick" and the "tack" of a watch, which is ideally zero (if you imagine a running watch sounding tick-tack-tick-tack...).
Cheers,
Axel
P.S. perhaps one of the native speakers can explain it better