For transaction imports, corresponding with huge giving campaigns (Giving Tuesday, End of Year, 401 gives-RI) we import a large number of donations leveraged by our board members. This includes hundreds of donations with different fundraisers, and it would be extremely time consuming to enter this information manually. We would prefer to track fundraisers for donations, because we often write reports/ submit grants that require us to identify how much our board members have contributed to our fundraising efforts.
I'd expand this to include updating transaction records through import. In this case there is the importing of fundraisers, but I would also note that custom fields can be created for transactions. It would be extremely useful to be able to import values to those custom fields instead of manual updates.
More broadly, having the ability to perform updates to clean/fix/capture missed data would be helpful, in transactions and across the board to all objects.