For other base-running. For example going from first to third on a single. And while it does not seem to affect fielding, though others here will know better, the SIM will default the OF with the highest speed rating to Centerfield. Though I am not aware that it affects range per se, so it may be pointless there.
But certainly it can make some difference for the offense, even without base stealing, and may be a good reason to put "Hit and Run"on the "Aggressive" setting. Often in the box score summary when you have fast runners you read that they slid in safely under the tag. Though I have not seen this quantified anywhere.
But the other posters here are correct - what really matters for base-stealing is first the SB/CS ratio, which you can search for in the draft center, and second the number of attempts. For example, 20 SB is fairly meaningless: you will find a slew of deadball era players with 30 SB and you think Wow ! and then find that they were caught stealing 29 times. You need at least a 67% rate of safely stealing out of attempts for base-stealing not to be counter-productive. And you want to search higher than that, at least 70% at a minimum.
But of course two players with the same SB/CS percentage (and speed rating) may be very different in reality: player A stole 4 bases in 6 attempts, which is a very low sample size, so the percentage is not that meaningful; while player B stole 44 bases in 22 attempts, which give you a much better idea of how good a base stealer they are.