One of the options within Control Panel, Screen saver is to indicate whether to power down all hard drives after XX amount of time. On a desktop, is this practical, does it prolong the life of the drive by reducing wear and tear, or is it primarily for battery conservation in laptops?

...or is it primarily for battery conservation in laptops?

That's what I consider it to be. In my experience power management features on desktops have historically not been as robust as those implemented in laptops. Depowering the drive(s) may result in the computer hanging or freezing when the drive(s) spin back up. There's also the nuisance of the time lag for usability waiting for the drive to power back up.

There might be a case for increased longevity through the reduction of the number of thermal cycles the drive experiences over its lifetime, but I don't have any formal studies to cite. The power savings is a definite positive.

So, if power savings is important to you, try it to see if your computer will work reliably with power-down enabled. I've never had one that did to my satisfaction. I leave my drives run 24/7. If they fail before the warranty runs out, so much the better; beats failing 6 months out of warranty because I coddled them.