That would be a lot of political capital used for what? Why would Biden help Trump in any way?
I would think of it less as "what could Trump give Biden" and more of "what could the MAGAs in Congress give Biden." Chances are good that a second-term Biden will need to work with at least one Republican-controlled chamber in Congress, and something like this might be what allows him to do something as simple as pass a budget.
For better or for worse, Trump is almost literally the second coming of Jesus to a large number of Republicans, and commuting his sentence might be something that provides Biden some leverage that could allow his presidency to function. Again, not saying that's anywhere near an ideal outcome (I'd much rather see Trump rot in prison), but it wasn't an outcome I'd previously considered and one I could see happening.
"I love all my children equally." - Lucille Bluth
earlier that day...
"I don't care for GOB." - Lucille Bluth