I have a pretty good idea of my running and biking paces. I know what cadences I can manage, what speeds I can hold and for how long. This means that I can manage my pacing using a few pieces of data, usually split times, cadence and heart rate.
With swimming I've been pretty much blind. Part of that is because I pretty much am blind, so unless I've put on contacts on the way to the pool, I can't see the clock on the wall at the pool. As a result, I don't really have a good sense (other than a vague kinesthetic one) of what stroke adjustments, kick adjustments and position adjustments do to my speed. This is a problem given how much of swimming is based on technique.
I've been using the watch for a few workouts, and it is helpful. First, it is reassuring. I'm more consistent than I thought, both with my lap times, my stroke count and so on. I'm not fast, but if I'm consistent, I can make adjustments and measure them. Second, if I make an adjustment for a set, I can see pretty directly how it has affected the various measurements. Do kick turns help? What about bilateral breathing? What about different breathing patterns, etc.
Okay, so it's another opportunity to be OCD, but that's a good thing right?