On list pages and other forms where records are displayed in a grid, the user can 'personalize' his view. He can add or remove columns, change the order of columns, etc. He can also save the view that he created. That is a very nice tool!

However, on lookup dropdowns, this tool is not available, for example this one:
Go to Module: Accounts receivable > All customers > (fasttab) Addresses > (button) Add
then in the Zip/postal code field, hit the down arrow.
The form you see now is a lookup dropdown, which you can't personalize.

So, the idea is simple: allow personalization on lookup dropdowns,