Some reasons for something in a fact description vs a fact note or vice-versa:
1) Reporting. The Genealogy (Register) Reports (ahnentafel or descending) contain an option under "items to include" of "fact notes" - yes or no. You can select facts individually to include in the report. Depending on your work habits, fact notes may add a lot of volume to your report; while adding the description field of a fact may add only one line per person.
2) Exporting. All facts will be exported in an export for the individuals selected, but you can choose to not export Private Notes (using the "Lock" icon).
3) FTM's standard filter - which can be used to see people in your index, or can be used to select individuals in a custom report (and some other). You can filter on text in notes, but it is easier (ie less memory) to filter based on a text string in a fact description.
So, if you think you might ever want to group a dataset of people in your database that share some trait - I would put it in a fact. If it is a piece of info that you might want to include in reports, but not include all of your fact notes, put it in as a fact description.
Another reason is that a Fact Note is somewhat "hidden" from view when viewing the person in Person View. If it is something you want to make visible in person view, you may want to put it in a fact description - or at least type the words "See Fact Note". Otherwise, the fact looks "empty" and not used.