Drum triggers alone are merely piezo crystals that create a varying AC voltage depending on the force in which they are struck.
You don't necessarily want to hit them directly, more so they pick up the impact force transfered from the drum head or shell (depending on the type).
I would assume you have seen some electronic drum sets ..... Same deal, only with acoustic drums.
In both instances, the triggers require a module (or brain, if you will) to transfer that varying AC voltage to a velocity sensitive switch, so to speak.
That module can then assign a sound to be played that dynamically matches the velocity of the hit each time that so called switch closes. Most modules also have the ability to translate that information into MIDI data.
Actually that's kind of backward, as most modules create the MIDI data first and you assign the note to be played according to that data.
They can, in a way, replace microphones provided you have a module to create the MIDI data and accompanying audio or MIDI sequencer and audio samples/sound fonts.
The benefits of triggering acoustic drums is of many....
Be it less than stellar sounding drums, poor quality mics (or no mics), inadequate room acoustics etc... you can sequence the MIDI data and use better sounding samples or sound fonts.
You can use a mixture of both the mic'ed acoustic drums and MIDI samples for a whole new flavor of sounds.
Depending on the gear used, this can be done live as well as in the recording environment.
