You can also get a stereo effect on a mono channel if you use the send. Depending on your DAW, putting a stereo effect on a mono insert will only give you the left channel of the effect.
Sending a snare and three toms to one reverb sounds different (and better) then having separate instances of the same reverb on each track. Part of that comes from the tracks interacting with each other in the one reverb.
Also, if you put a different reverb on each drum the reverb for each individual tom will be panned to where the tom is panned. Since a drum set is normally treated as one instrument playing in a single space and not a collection of instruments playing is separate places, it would make sense to send them to a single reverb.