There are at least two reasons for an SS MTB.
Firstly, it can be a very tough street bike - build it with narrow rims, fast slicks, and a long stem or even pursuits. I almost bought a Stumpjumper that had been built as a winter training bike for road racing - it had the rider in trials bike position, 26 x 1.1(?) inch Gatorskins and on the flat it could move faster than the owner's high end carbon racer. Less extreme examples with cut down flat bars and 1.5ish slicks are popular as courier bikes.
Secondly, as an offroad - it makes things harder, which for some people is more fun.
Search for Sheldon Brown's site, he goes into everything singlespeed in detail, including inches for MTB and road bikes. Obviously an MTB built as a psuedo courier bike should be geared like a road bike.
Oh - and the chain is much less likely to come off a correctly built single, so good for a jump and stunt bike too.