Repositioning/ferry flights

Can somebody answer this question for me?!

Delta keeps flying an empty MD88 overnight (PVD to DTW) and then the aircraft sits at the same gate all day and then ferries the aircraft (DTW MSP) where it sits for 24 hours before ferrying it (MSP DTW).

Can somebody please tell me why they keep ferrying this aircraft back and forth between city pairs?

Curiosity is really the better of me and I’m intrigued.

Delta has large TechOps at KDTW and KMSP. I’m not sure about KPVD. The flights could be (non-revenue) for internal operations. What are the flight identifiers or tail numbers?

Delta Flight #’s: 8831/8912