What the others above said, summary.
Reducing a recoil spring weight "speeds up the slide cycle speed. This can improve ejections tha land short and speed up the reset and the second shot speed
Iif your are shooting weak/light load ammo, a lighter recoil spring will improve the ejections and more reliably feed the next round.
If you are shooting hot/.strong load ammo, a lighter recoil spring can some battering of the slide and slide release.
It will also toss the brass into the next time zone.
Ironic witch is right in the a lighter spring will give more felt recoil by speeding up the slide, tending to string fast shots UP. The stronger springs can reduce slide speed and felt recoil, tending to string fast shots DOWN. The shooters grip also plays a part in these results
This seemingly small one spring change can effect a lot of the pistols characteristics. Some are useful.
Reloaders can make use of this by tailoring the ammo along with the spring for optimum effectiveness for " that" pistol for THAT shooter.