Every 'normal' integrated amplifier has a pre and power amplifier inside
The PRE stage takes a tiny delicate input from the sources, cd, streamer, turntable, radio, etc and switches it to the outputs
This has to be done without losing any of that tiny signal of course, and it also has to do the first amplification and volume stage
The POWER section takes the output from the PRE and does the heavy lifting to bring it up to the correct voltage to drive the loudspeakers
At a certain performance level keeping these two distinct parts separate gives better performance
Many preamplifiers now come with streamers built in. This makes sense from both a technical and a cost basis and has also sparked debates about digital vs analgue voliume controls.
If you're system is mostly streaming then the latest technologies to do the volume in the digital domain really pays dividends.