Tuning drums is not just for matching the key of a song. You also tune drums to change the freq and tone. If you want a tight, snappy snare that cracks loud, you pitch it up. If want more of a bassy, loose/thick snare, pitch it down. You dont always need to find a new sample. One good snare sample can be used over and over and sound like a different snare just by pitching it up/down.
If you owned a real drum kit, you wouldn't have to go and get a new snare drum every time you wanted a new sound. You adjust the sound by tuning it to give you a different snare sound.
Its good to tune your drums to the pitch you want before you start EQ'ing as well, so you get it in the octave of the frequency range you want first, then just use the EQ for any fine tuning. In other words, if you think your snare sample has too much bass and not enough treble, you dont have to just crank the treble and turn down the bass. You can just pitch it up to higher octave. Then use your EQ to fine tune it, if nec.
In the early drum machines like the Linn and DMX, they only had 1 sample of each sound. Pitching the samples was the way artists like prince/rick james could use the same kick/snare sample for every song on an album without them all sounding exactly the same.