HiI play music for ritual dancing (sword) on a regular basis. In general, one tune is required - whether it be AB or ABC in structure. However, this gets boring - partly for the listeners but particularly for the musicians who may have to play the same tune through 20 or 30 times.
To alleviate the boredom, the musician will choose another tune of the same type (jig, reel, polka etc.) which appears to fit and can be easily changed to from the first tune. After a while, the set becomes fairly fixed - especially if there is more than one musician because then the sequence can be memorised and only the word "change" used.
There are other reasons for putting tunes into sets, though. Hornpipes are often grouped into sets called "breakdowns" to allow a musical progression. Some tunes (e.g. the "bacca pipes jigs" were composed as a set in the first place.
Cheers!
IanC