I know that dances were customarily danced in half-hour sets of two dances with the same partners. My questions is "why?" I am interested in the thinking behind the custom.

• Was a half hour considered the best length of time for two young people to be engaged (not too short, not too long)?
  After all, a practice of forcing dancers to change partners would also have prevented dancing too long with one person; so that is likely not the only reason for sets.
  
• Did a set combine a slower dance and a faster dance thus providing the dancers variety?
• Were breaks for refreshments scheduled relative to sets that made them logical units?
• Some other reason(s)?