Not a silly question at all.
If you are just running a standard motor I wouldn't say it is essential but they do help protect your driveline from wear and tear. When you move onto more powerful motors, they become more necessary.
Basically, without a slipper clutch, every time your motor turns, your wheels turn. There is no 'give' in the driveline. This can be a problem if you do a lot of jumping because as you get airborne, your motor turns very fast (not resistance) and your wheels spin really fast but when you land and they make contact with the ground again, the wheels are suddenly much harder to turn and this creates a lot of stress in the driveline. Something has to give and one day it will be a worn outdrive or a stripped gear.
A slipper clutch is made up of a couple of plates pushed really tightly together. Under normal load it acts as one solid unit but under stress - such as hard acceleration or landing from a jump with wheels spinning frantically - the two plates 'slip' against each other to absorb the stress.
Does that make sense? There is a bit more to it and they are adjustable to give more or less slip but that is the basic rundown.
I ran a Dark Impact for a year with standard motor and no slipper and I can see a little wear on the outdrives. I have upgraded to a 23 turn BZ motor and installed a slipper for a bit of protection. Seem to work.
Yuley