because it's pretty. not sure, but i know many many years ago when the Police carried their spare ammo on their duty belts in loops, the chemicals used in the tanning process for the leather would corrode the brass cased ammo if left for too long. the nickle cases would help with this. some rifle cases are nickle plated buy the way.
Nickel plated cases are far better corrosion resistant cases than brass cases for police and military use. The high dollatr Hornady TAP rifle ammo are all made with nickel cases. Also nickel is much smoother, less porous and is more slick for automatic actions. However, you cant get as many reloads out of the cases. Nickel cases are work hardened much faster than ordinary brass. But they sure look cool !!
I use nickel cases when I go to big matches that don't let you police your brass (cowboy action), because they tend to split/rupture before "brass" brass does. Even with my blackpowder loads. I figure it's just a matter of one or two more loadings with those anyway, and I can get 10+ out of the brass ones!
It has its intended purposes (corrosion resistance, slicker surface for more reliable ejection, etc) but I mainly use it to identify my 45-70 loads at a glance. The lighter deer loads get brass, the heavier ones for hogs get nickel. I use it occasionally in pistol calibers too because I can clean it and give it to the wife for jewelry.
When there are more than 100 competitors (I've shot in matches with 720 competitors, 12 stages, 3 days!), running through 10-12 stages in a couple days, it really slows things up - takes too long to spend time searching them out from around the stage props.
Usually, the match directors will have the local boy scouts or 4H out to police up all the brass each day after the range is shut down, then auction it off in lots at the awards ceremony with the kids' group getting the money.