Putting more than one song together to make a bigger structure is a standard part of the arranger’s craft. There are all kinds of interesting things to think about the how you join the songs together, but today my interest is on the more fundamental level of how you choose them.
I think about medleys in two types. One is a collection of tunes that share a common origin or set of associations. Say, selections from a show, or songs by (or made famous by) a particular artist or group, such as my Madonna Medley or Meatloaf Medley. I tend to think of these as show pieces, useful as they offer a longer span of musical time without a break than single songs would, in much the same way that classical concert programming places more substantial works as focal points amongst the shorter items.