Phono preamps do primarily two things.
First, the signal generated by the phono cartridge is quite small. It is limited by what the needle can do when it vibrates in the record groove and moves a tiny coil in a tiny magnet (or a magnet insided a coil). This is a much smaller signal than what comes out of a CD player, a tuner, or a tape machine, all of which have the benefit of built-in amplification stages. The phono preamp amplifies the signal from the cartridge so it is about as big as the others. Accordingly, the line level preamp receives a signal that is usably loud.
Secondly, the phono preamp reverses the RIAA volume curve treatment. An interesting truth about the physics of putting sound directly into a record groove is the low notes require huge waves in the vinyl and the high notes are represented by really small waves. This would be hard for the cartridge and needle to handle. As a result, in the early days of records, the RIAA (Recording Industry Association of America) set a standard where the low notes would be put on records at reduced volume, to make the waves smaller, and the high notes would be put on records at increased volume, to make the waves bigger. This would allow the needles and cartridges of the day to satisfactorily capture both highs and lows. When records are played back, this has to be reversed. The phono preamp amplifies the low notes and attenuates the high notes so that the music sounds like it should.
Depending on the phono preamp, a particular unit might also do things like adjust for proper capacitance loading for the cartridge.
As you can see, the phono preamp is not optional, if you have a record player. Sometimes phono preamps are built into preamps, and you don't need a separate box. This was more true in the old days when records were the primary way people played music at home. Nowadays, record players are relatively rare (sigh), and many manufacturers don't bother to build phono preamp stages into their preamps. So we have to buy stand alone units if we want to play records.