This is an excellent question. As answered well in the posts above, a preamplifier simply adds gain to the source signal such that it can be of sufficient strength for the power amp to add more gain to boost the signal to the level needed to drive speakers.
There are indeed lots of discussions about how a preamplifier can improve the quality of amplified sound. Such as it is, the quality of the gain supplied by the preamp impacts the sound more than the quality of the power amp. That can be overstated, of course, because quality power amplification is needed. However, the preamplifier can make or break the sound as the signal goes downstream.
This principle is demonstrated in how often we see (actually hear, sorry for the mixed metaphor) that the flavor the preamp imparts, say a tube preamp, is more significant than the power amplifier. Some folks may run a tube preamplifier in front of a SS power amp and hear more tube sound than a SS preamp in front of a tube power amp.
Historically, where vintage preamps and receivers always had a phono input, now the mainstream gear is often line stage only. This is because a preamp has to do alot more work to amplify the minute signal from a TT cartridge. Sources such as CD or DVD players may have a signal of sufficient strength to drive a power amp on their own. So less is demanded of a line state preamp. Some preamplifiers still offer phono inputs, but that's the minority report. Some folks use stand alone phono preamps to use with their TT setups.
One thing that we see now in some so called "digital" switcher amps is that the gain is accomplished in a single stage such that a preamplifer is not needed. The source goes straight to the amp to the speakers without two seperate preamp and power amp stages. Pretty cool.
There are so many preamp and power amp options out there, SS and tubes and such, that some care does need to be taken to confirm that the preamp is a good match for any given power amp. Knowing what impedances and db of gain are needed is beyond my depth, thankfully there are folks on this forum who offer wise advice on what's a good match or not.
One other thought, that is most interesting to me, is the synergistic effect of gear combinations. There are some preamp and power amp matches that produce sweet music beyond our expectations. Others do not complement each other well at all. This is another benefit of the base of knowledge among forum members. There is lots of experience to draw from so that others don't have to reinvent the wheel to find good combinations.