When you already have edit privileges, then you can choose to leave a summary comment when you're just retagging. You don't have that ability when all you can do is retag questions, and depending on which site you're on it can be a pretty steep climb between 500 reputation and 2000 reputation. I propose that people who are retagging are allowed the ability to leave a summary.
Now, not everyone leaves summaries, and in general the nature of a retag can be implicit without needing explanation. But I think it can help in some cases, especially when there can be disputes about the tagging. Being able to leave a summary on a retag allows the retagger to immediately explain themselves, rather than waiting for someone to rollback the change and subsequently commenting.
It's more of a convenience factor, in a way, and naturally only gives the benefit for an ideally temporary period of time (as when you hit 2000 rep, you can leave edit summaries anyway). But considering we already do give the ability to retag at this stage, I don't think it is particularly harmful to allow people to leave a summary. After all, if you're doing a mass-retag you can always just skip leaving a summary.
Side effect - Notedly, this would allow one to mitigate certain issues.