My very crude observations so far is, that a lot of those sub 1 sat/vB fee rate transactions are data spam, where it might not be important when it is confirmed. As mempools eventually dry out during a day or weekend but multiple times during a week these days, it makes a lot of sense to me to use sub 1 sat/vB fee rate.

I would do the same for transactions that aren't urgent. Why waste precious sats?