Preamp for vocals? Do you mean to add FX or something when playing live? I'm just curious cause never heard of that before and I do a lot of singing both live and for recordings...
Since the smarty pants peeps have not posted, I will give you what I know and understand. Disclaimer, as seen on earlier posts: I am not an expert!!
Ok, first of all, you have been using preamps when you are singing in a mic, and there is a cable and it's plugged in to a mixer or recording interface. So, they are there, just not a thing that you necessarily think about. Those mixers and interfaces have a mic preamp behind each of those xlr channels but, since they have to make a profit, the 8 preamps on my Tascam interface are not amazing. They're not horrible, but they are not pristine, clean and amazing. Same with mixers, bunch of preamps in there. But, those preamps are just set and can't be adjusted. Microphones differ in what they need to sound their best and using a preamp, as I intend to do for the next recordings of songs, will let me have some control for boosting the output of the mic, which is pretty small, up to a good level going in to an interface. But the other part of this is that while I have preamps good enough for recording in my interface, are OK but they don't ADD anything. Some Preamps, due to their architecture, actually are not totally transparent. They ADD some color. Sometimes it's color that people dig, sometimes not. But when you talk about great sounds, a lot of them were due to the preamp they were going through.
This is not perfect or complete but what I am interested in something that can give my a bit more girth, size, presence on vocals. I am hoping, that the incremental progress I have made recording/mixing and so on, will include better vocals. Fingers crossed.