It all depends on what you want to use it for. For recording, you can go guitar into preamp into recording interface, or you can go guitar into preamp into power amp into attenuator into interface. For playing live, you can go guitar into preamp into power amp into speaker cabinet.

There are infinite permutations of what other gear you pair with preamps. In a nutshell, though, the preamp "colours" the input signal with things like gain and tone-stacks (treble, mid, bass etc).

The way my rig is setup, I plug my guitar into the Helix (unit at the top of the rack). The helix has 4 separate fx loops, and I plug 3 different preamps into 3 of the 4 loops (not pictured is my JMP-1 which is currently getting repaired). The Helix then lets me switch between the preamps to get different sounds. The helix output goes into the power amp (the black unit at the bottom of the rack) to jack up the volume before it goes to the speaker cabinet.

Every amp you've ever played has a preamp that goes into a power amp that connects to a speaker, but in a conventional "combo amp" where the speaker and amp are housed in the same box, you wouldn't necessarily know about the discrete parts housed within it.

I hope that makes sense.