Essentially the pre-amps’ primary function is to boost the Mic output signal so that it can properly drive your souncard or multitrack recorder.
Now we’re talking from just a few millivolts for a dynamic or ribbon mic to around 1 volt in a 600 ohm load….which is a healthy line level signal.

In the big picture of your final recording , how this is done can be quite important.

For example , even though the mic and the sound elements may be of high calibre and quality , their sonic goodness may be impeded by the fact that the preamp has a less than full bandwidth or has a noise floor that cannot be improved upon because of the gain structure.

It’s like looking at the trees though a dirty window.

These imperfections will be re-inforced and added together everytime you overdub and build on your previous tracks. It also limits your final Compression/Equalisation possiblities because of that possible build up of noise and shit.

A lot of commercially available units/preamps seem to suffer from poor Transient response …..
The cheaper ones will be slow and lose resolution in top end detail.
This is due to limitations in supply rail demand and excessive circuitry…usually chips which when added together slow the audio path down.
Other units may have excellent transient response but consequently recordings may become too dynamic or brittle when layered .This can make mixing quite difficult.

With the right preamp and setup , the recording almost mixes itself.
Those who have been there will know what I’m talking about…..it’s a blissful feeling.
The ease of the session seems to entail even more creativity from all the participants.
That’s why the right pre-amp is really important…..

We here at ~Sebatron~ strive for that. ;)

Some audio engineers and electrical engineers will view this question from a very bland point of reference.
-Mic level is low and I need it to be higher so I add some gain. Add gain with as little noise as possible and with a degree of headroom that meets ones needs.

News gathering ,Drama Interviews and the Film shoot are the a twist of the same basic principal. This is why you might find less variation in the choices of Mics and Mic-pres in the Broadcast and Film and TV industry.

When making music I think they is a huge scope for variation. I can only parallel this to Guitars and Guitar Amplifiers and the hundreds of combinations and settings you can have. Even though there is a relatively small number of basic Guitar types the variations are enormous.

If it sounds good ... print it.
If you use a variety of methods and they all sound good ... mixing gets much easier.