I know there is always that guy that poses this question when HDDs are brought up, but I'll do it anyway...
Why, in this day and age, where SSDs have the capacity and the low pricing they have, would you ever want an HDD in your main build?
Let's not even put SFF in the equation, b/c that only makes this more rhetorical than it already is... it just makes no sense to me. If you have a ton of photos, film, or music... build a server PC away from your desk to save on the horrific spinning noise and case bulk, and leave all the quiet sex-appeal on your desk to the SFF cases with M.2 drives and SSDs.
I'm not telling anyone how to live their lives or anything, b/c you do you... but I have a hard time even recommending SATA SSDs anymore b/c M.2s are so well priced and have decent capacity, and all without the cable mess lol.