To be more specific, an amplifier consists of 3 main sections. The preamp, poweramp and speaker cabinet.
The preamp is where the signal is "shaped", eq-ed, etc. The low signal strength from the instrument is amplified strong enough to drive a poweramp here. It comes before the poweramp, hence it's call the PRE amp.
The poweramp is where the electrical power to push the speakers come about. Primary purpose is to provide energy to drive speakers, hence the name POWER amp.
The last part is the speaker cabinet. Not just a speaker, but an enclosure ported and build to a certain volume to match it's internal speaker. Each cabinet to tuned to a certain freq, and built with specific material, designs to optimise it's response. Not simply just a box with speakers stuffed in. Alot of ppl dun realise it, but the speaker cabinet makes as much difference in your tone as any other component in ur setup.
Combo amps are basically all 3 in 1. with the preamp and poweramp all built into the speaker cabinet.
A head is a preamp and poweramp built into one unit.
It's all about flexibility of being able to mix and match. If u purchase em seperately, u get a modular system, which u can use with other speaker cabinets, preamps, etc. Ease of upgrade as well. E.g. if u decide that u need a louder amp, just get a bigger poweramp.