they amplify, and they attenuate.
using simple round #'s, a bit of history. Legacy equipment/terms are in use but ....
1. original mono sources (radio tuners, ceramic phono cartridges) provided strong enough signals directly to amplifiers, let's say 1 volt. This strength is called 'Line Level'. (newer, like CD's provide more, i.e. 2 volt or more). Pre-amps did not exist.
All sources producing 'line level' can go direct to an amp without a pre-amp, the only consideration being volume control (attenuation).
2. early stereo, tapes in 1956, your system being mono to begin with, simply add another mono amp and speaker, both amps receiving 1 volt. LP went stereo in 1958.
3. enter low voltage stereo signals. a. tape head (low signal direct from the tape/tape head). b. phono magnetic cartridge signals (phono low). pre-amps were ADDED, to pre-amplify the new low signals up to 1 volt, then into the original amps. Thus they amplify.
4. both new low level signals, tape and phono ALSO needed/still need 'equalization'. Standardized recording/playback systems were developed, NAB for tapes, RIAA for phono.
Phono as example: (tape recording/playback a similar process, a different 'eq' curve).)
Phono initial physical recording cuts lows (minimizes width of groove needed for bass), AND boost's highs (gets signal further away from noise, like later Dolby does).
LP. Long playing. Cutting the bass grooves allowed more grooves, thus 'long playing'.
Phono Playback needs an equalization circuit, to boost the lows and to cut the highs, before it goes to the amplification portion of the pre-amp, to get up to 1 volt to go to the amp.
Phono 'moving coil' came later, weaker signal, needing a pre-pre amp so to speak, to get it up to phono low, then to pre-qamp's eq and boost uo to line level for the amp.
'Tape Head', signal directly from the tape, is low level, needing both NAB eq and pre-amplification, thus separate inputs.
'Tape' is line level, already equalized and strength boosted, by the tape recorder/player's electronics, therefore it can go in any tape input, tape loop, or straight to an amp (again, how control volume?)
5. Attenuation (volume control): various sources produce a variety of signal strength, they need to be 'adjusted' prior to going to the amp, thus a pre-amp volume control. Implementation varies, cut/boost/bypass, ....
McIntosh and others provide 'trim' controls, which individually 'pre-adjust' those low voltages 'to match each other's signal strength' minimizing the need to adjust the pre-amp volume (switch inputs-get same volume).
6. Headphones: some pre-amps have separate headphone amplifiers. Never goes to the amp.
7. Rare Pre-amps, like McIntosh C26, use the headphone amplifier circuit, say 10 wpc, to directly drive efficient speakers. Like future receivers, they offer 'main, 'remote', or both simultaneously.
http://www.berners.ch/McIntosh/Downloads/C26_own.pdf
8. Add a tuner to the pre-amp, like my new to me mx110z
http://www.berners.ch/McIntosh/Downloads/MX110_own.pdf
9. Add multiple inputs, switching, volume control to the amp, get 'Integrated Amp, like my new to me Cayin A88T. (I needed MK1 to get 16 ohm taps from the transformer, rare these days).
https://www.elusivedisc.com/CAYIAA88T.pdf
10. Add a Remote Line Controller, inputs, volume, balance (I love squeaking the balance speck by speck from listening position), tone, .
https://www.ebay.com/p/Vtg-Chase-Tech-Rlc-1-Remote-Line-Controller-Upgrade-Stereo-to-Wireless-IOB-J588/1424512686
What a complicated web I have built for myself, selling my Digital pre-amp and amp and getting tube pre-amp, tube tuner, tube amp, all with various methods of control, direct and remote.
Oh yeah, you young folks need to deal with modern digital sources, and modern headphones.
