It would be really useful to sort the parts in an inventory by their quantity, i.e. the parts of which there are the most copies show up at the top (or bottom).
Use case #1: When I check a set for completeness, I sort my bricks on a table. I then take the inventory at Rebrickable and go down the list, throwing the parts in a container. If I could sort the list by quantity, I could speed this up by first removing the largest quantities of bricks from my workspace. They are often easiest to find (except for the last ones 😉 ). If I then, for some reason, have to clear my workspace, I don't lose much work by throwing all the unique or low-quantity parts back in a bin to continue sorting them later. I can easily record that I have gone through the list up until parts with quantity X and continue from there later.
Use case #2: Find out which parts are most numerous in a set.
Use case #3: ...