Unlike line level audio sources you connect to a stereo (DVD/CD players, tape decks, TV audio, minidisc, etc.), the output from a magnetic cartridge installed in a good quality turntable is MUCH lower, and requires an additional stage of amplification to bring it up to the same volume as the other sources you listen to through your stereo.
This additional amp stage, the phono preamp, is built-in to most older receivers and amps, allowing direct connection of a turntable. However, newer stereo equipment (including virtually all mini-systems and home theatre units, as well as many stereo receivers and amps), have NO phono input (this because records and turntables are supposedly obsolete in today?s world dominated by CDs and DVDs).
In order to utilise the inputs such units DO have (Aux, Tape, Line, Video, CD, etc.) to connect a turntable, you need to first pass the signal through an external phono preamp to bump the level.
The same level increase is needed if you?re connecting a turntable to a computer sound card?s line input so you can make CD-Rs from LPs again, the external phono preamp provides it.