As a dance musician and caller of some 40 years the answer is pretty obvious and Tootler hit on it when he/she used the word variety.
Variety is the spice of life. When I used to go out 2-3 dances a week playing the same tune over and over again could get pretty boring, so one solution was medleys of tunes. We could sometimes get through 6-7 tunes in one dance. In the days before recording when village musicians needed the income they would be travelling round the area trying to get as much work as possible and the same conditions would apply. You only have to look at the wealth of tunes in those old 18th and 19thc ms tune books to realise this.