Multiple short sets vs one long set

What's the theory behind multiple sets (particularly swimming sets) in triathlon/ironman training vs just one long set every time? I'm planning a full ironman for next year, I've only done a half and then a few olympics and sprints up to now, and I'm working on a training plan. Up to now, I've always just done one swimming set (plus a short warm up) every time I swim (my normal training has been to work up to the distance that I'll be doing for the tri by a couple months out, swimming further each session, then slowly increase my speed swimming the full distance for the 4-5 weeks leading up to the event plus a taper week). I usually swim open water, very occasionally in a pool, but usually lake or ocean. I guess I'm wondering if there's science behind sets, or it's more of a pool thing?