The purpose of a preamp?
There are multiple reasons, depending on your setup.
1. Selecting between multiple sources.
2. Some sources don't have their own volume control and most power amps don't have one either.
3. Some power amps don't have enough internal gain to put out full power from a line level source and/or not all line level sources output 2 volts.
4. Gain can also be an issue with some recordings. LPs are a classic example where records with 15 minute long sides could be recorded louder than those with 20 or 25 minutes on a side. I've also seen CDs recorded at unusually low average levels. For those recordings you may need more gain than the power amp along provides.
5. The output impedance of a source and the input impedence of a power amp may not be a good match. Or, as already noted in a prior reply, interconnecting cables, particularly if longer, can adversely affect the sound quality.
6. A preamp can also imprint its own sonic signature on the sound quality. You may like the way a particular preamp sounds versus no preamp.
Even if your particular setup doesn't have any of the above issues, the only way you will know what you like best is to experiment.