Why a 9x19 revo, you mean?
(Don't feel bad, I used to think the same thing.)
Turns out the 9mm performs better in a snub than a .38 spl (115 - 147 gr.) and isn't as loud and obnoxious as a .357. More efficient.
At least, that's my answer to "Why?" Now, some folks might like the 9mm revos as a back up to their 9mm autos, or because they're set up to reload 9mm, or because they don't want to have too many calibers to deal with, or because it's dirt cheap to shoot.
Oh, yeah. I like that last reason, too, myself. I shoot my 940 way more than any .38 snub I've owned (I used to reload .38s, but what a PITA for lotsa rounds), and every time I pull the trigger it's with a full house combat-power load under the hammer.
Gotta love 'em!