subwoofers for music

Pretty much a beginner. I made a set of DIII's back in 2003 and now have the DIY itch again. . I have looked at many designs, and my first question is about picking one to build. I am interested in speakers primarily for music (mainly classic and hard rock). I do not care about home theater sound such as the explosions and dinosaurs walking around.
I have seen some designs that began as a two way and then developed into a 2.5 or three way by adding a third driver and a larger floorstanding cabinet. This makes sense as a way to add to the low end. My confusion comes from many of the two way bookshelf designs being demonstrated with one or more subwoofers. What is the difference,sound wise, when comparing a larger floorstander with a bookshelf and a subwoofer?
Thanks much
 
Hi perhellion,
Integration and a flatter frequency response given reasonable conditions. Also the box is taller, raising the tweeters and mid drivers closer to ear level.

I own two pairs of PSB Stratus Gold speakers which are basically a two way on top of a sub box ( a little more complicated than that). There is no need of a subwoofer with these speakers and I use one pair for music, the other in a surround system.

Sound is sound, and the more accurate it is, the more enjoyable it is as well. I also own a pair of Klipsch reference towers that use two 8" drivers and a Tractrix horn. These are also very good sounding and their height fires the sound up where I can hear it pretty well in bed. That and they are fun, being 98 dB/watt efficiency. These do not have as low a frequency response as the PSBs do, but they cover all natural instruments and for music, I am satisfied with them. For theater I might use a large subwoofer with them. Maybe.

-Chris