Full frameworks have code inside them for virtually any and every need - be that for galleries, different types of menus, accordions, sliders, cards etc etc etc. All of this code is loaded onto your page whether you need it or not. If you use some of these things then you will be getting the use of some of that code. If you don't then all of that code is sitting redundant on your page and causing your pages to load slower than they need to.
Additionally, thanks to all of the fantastic stacks available via the RapidWeaver Community, many people use dedicated third-party stacks for things like sliders, menus and galleries etc. They do this because they are better and more flexible than the framework version - but doing so it adds yet more code to the page and makes the framework code increasingly redundant.
Source was born because I personally reached the stage where I was effectively using an entire framework to set my colours and my fonts. Virtually every stack that I added to my pages were third-party stacks that offered me the power and flexibility that I needed.
Source strips out all of that unnecessary code and lets you use the Source stacks along with your favourites from the RapidWeaver Community.