As you noted, the preamp preamplifies the signal before sending it to the amp. It also allows switching between sources, the inclusion of a phono stage, may provide a balance control, another set of preamp out outputs, etc. Any of these functions can also be included in an integrated amp but sometimes manufacturers save some of these functions for their separates.
I think it's safe to say that manufacturers build their more expensive separates to sound better than a less expensive integrated. However, in a given system the improved sound quality of the separates may not be large enough to justify the higher price or may not even be able to be heard at all.
I'm with jmcgrogan, if you love the sound of your current system stick with it until you don't love it anymore. Many people spend a lot of money and don't end up loving their system.
I think it's safe to say that manufacturers build their more expensive separates to sound better than a less expensive integrated. However, in a given system the improved sound quality of the separates may not be large enough to justify the higher price or may not even be able to be heard at all.
I'm with jmcgrogan, if you love the sound of your current system stick with it until you don't love it anymore. Many people spend a lot of money and don't end up loving their system.