There are two reasons I like to swim long sets.
1. I find it relaxing.
2. It's only after I've been swimming continuously for at least 20 mins that I can really start to feel my stroke degrade. And at that point I can really focus on the adjustments necessary to keep things on track for long swims.
In particular I can really feel the strain on my right shoulder and my left elbow, also the strain on my lower back and the beginnings of fatigue in the core. And when I start to get that feedback I can really work on optimising my stroke to minimise these negative feedback points.