Why do people run one rail?
There are a lot of guys who use a single rail on the toe side, which they say works best for them, the main reason being for boardslides and lipslides (always forwards facing, so back boards and front lips) so they can sit on their heels to slow it or put more weight on their toe side which, with the rail, will not pitch them forward the way no rails would do on boardslides, if you get what I mean.
They also like to use it to lock in front smith or back feebles too.
Rail placement also has a lot to do with some tricks, as has been mentioned already, having the rail sitting in a bit means you can really lean a lot more on heels or toes to get the board up into combinations, eg boardslide up into hurricane or feeble easily. Rails sitting out almost at the edge will often be more for people who just want maximum flat width on the boardslides, but I also know some guys who ride wider boards and low or slim rails right on the edge so they still get a lot of deck contact but can lean on the rails more if they need to, which is a different take.
Rails are fun but if you are not used to them, you can slip out very quickly until you account for them, eg I don't usually ride rails but the couple of boards I have (second hand with rails already on them) I skate occasionally and it is fun but a bit scary for me, but damn you can slide for ever on them!!!