Triathletes find that they can make real progress by swimming in place. The optional Endless Pools underwater mirrors let them observe every detail of their swim stroke in real time. That lets them make immediate improvements.
With the rare opportunity to observe your freestyle stroke, you can build muscle memory. That is, you can connect what you're doing with how it feels. Sometimes, for instance, swimmers think that their arm isn't crossing over when it really is. Finally, you can know for sure exactly how it feels to swim with proper form.