Standard head unit has built in amp that power the speakers.
Bose head unit has no amplifier, so an external amp is necessary to provide a strong enough signal to power the speakers.
An external amp is a way to get bigger/better/more powerful sound as you have the space to fit the components in and the ability to dispose of excess heat. But it is also sometimes a bit of a marketing gimmick.
A third party head unit will usually have a built in amplifier that can connect directly to the speakers ; but in the case of a Bose headunit replacement the speaker cables don’t run from the head unit, they run from the (hard to get to) Bose amp. So what most people do (me included) is to use an adapter that connects the new head unit to the existing wiring which means the sound goes through an amplifier in the head unit, then a resister pack to reduce the signal and then through the Bose amp. This is generally said to be good enough, if not optimal.
Personally, I don’t like how mine sounds and am thinking about either using preout (low level unamplified signal) from the head unit through the Bose amp and see if that helps, and if it didn’t then I would remove the Bose amp and replace it with a third party amplifier.
Another option is to bypass the amp, use the output from the head unit’s amplifier through new speaker cables direct to the speakers.