Other than impedance matching and volume control?
Preamps also provide any gain needed to drive the power amp. This is handy if you have a tape machine, tuner or phono section.
In addition a good preamp will minimize or eliminate colorations caused by interconnect cables. If you've auditioned interconnect cables and heard a difference (for example, one does the highs better) then you know what I'm talking about.
If you get the right preamp, it can have less coloration than any passive system available on this account.