I keep count for maintenance purposes, selling or trading down the road, and should it need to go back for repairs for a particular reason I can give an accurate figure on how many rounds I've run through it.
I also keep a break down on what kind and how many rds of different ammo I've used. To try and watch for trends of any kind that a particular pistol may or may have with different types of ammo.
"And I saw heaven opened, and behold a white horse; and he that sat upon him was called Faithful and True, and in righteousness he doth judge and make war." ~ Revelation 19:11