Once they start to recover the first stage, they'll get a lot more information about how their systems really performed, as opposed to how they think they performed from the telemetry. You can do all sorts of testing on all the returned components, figure out how much of the margin of safety was actually used, and look for fatigue cracks that weren't quite bad enough to result in failure this time round. In principle at least, this could greatly increase the safety of future launches, at least as far as the first stage is concerned.