I've seen quite a few drummers put (duct) tape on their snare drums. Why do they do it? Does it reduce volume or change the tone? Can you use it for other drums, eg toms, as well?
It reduces volume, but usually the desired effect is to remove excessive ringing. If you listen to the snare on the St. Anger record by Metallica, you'll hear the type of sounds some drummers want to mitigate with the use of tape or other dampers.
This technique can be used on toms too. There it's usually not the high pitched ring of the snare, but lower frequencies that cause problems. There are damper rings and special gel manufactured for this purpose too.
I'll add pictures to precise the answer of Meaningful Username ;)
This is more for sound mitigation :
This is more for tone control (to reduce some harmonies) :
Since this answer is appreciated, I'll add that some drummers prefer (and advise) to use gaffer instead of duct tape to avoid adhesive residues. Duct tape uses natural adhesive where gaffer tape uses synthetic adhesive.
It is an attempt to control the ring or sustain of the drum. However, this can be accomplished with proper tuning techniques as well. Bob Gatzen did some fairly creepy yet informative videos on the subject. Play with your tunings! It's amazing what different sounds you can get from a drum with proper tuning.
Short answer: Lessen sustain and rid overtones.
Two of my friends add duct tape on their drums to shape the tone of the toms (for a mellow sound), but never on the snare (poppy sound). Without the tape, the sustain of the toms was way too long for the small area we record in. The duct tape acted as a cheap damper. However, I personally like the sound of Evans E-Rings more than duct tape.