Functionally: Volume Control is the most common need. Thus a Preamp is an attenuator, most combined with a gain stage. Passive is a switcher, attenuator with no gain stage, perhaps other features.
Pre-Amp, (the pre) (ignoring features, switching, tone/filter/modes/etc) are needed to pre-amplify low level signals up to Line Level, then off to the amp., i.e. Moving Magnet Cartridge’s low signal strength
Any Line Level Strength signal can go straight to the amp, BUT, where is the volume control?
’Back in the Day’: Amps were/are designed to receive ’line level’. Original Sources, i.e. Mono FM produced/sent 1 volt to an amp.
Stereo: you need another amp, stereo source like Stereo FM. Now two 1 volt out to two amps to two speakers.
Vinyl: original ceramic cartridges were strong signal, (remember RIAA EQ had to occur somewhere), originally within the preamp, now often a separate component: then Line Level Signal to Amp. MM cartridge signal strength, stronger than Ceramic, into MM Phono for RIAA EQ, and boosted to line level.
Pre-Pre Amp. MC cartridges, lower signal strength, in order to use existing MM RIAA circuits, needed to be pre-amplified up to MM signal strength, then to MM Phono EQ input, then out at line level. SUTs, i.e. Step Up Transformers are pre-preamp solutions.
Modern sources, i.e. CD Players, digital output ... can send much more than the old 1 volt line level.
Some Preamps, like my current and past McIntosh preamps, and prior Tandberg Receiver, let you ’level’ the various signals to each other, so there is no need to alter the volume control when switching sources, i.e. Phono 1; Phono Two; Tape: matched level controls, then off to the Primary Attenuator.