Prednisone helps arthritis too. It depends on whether the arthritic pain is inflammatory pain or not. If the arthritic pain is "mechanical pain" caused my wear and tear then prednisone probably wont help too much. However, if the arthritis has any inflammation involved then prednisone will work.

There are many autoimmune disorders that cause arthritis. Collectively they are referred to as "inflammatory arthritis." Rheumatoid arthritis (RA) is the most well known type of inflammatory arthritis. Prednisone was first used as a treatment for rheumatoid arthritis.The history of corticosteroids and prednisone in particular is amazing.
https://www.sciencedirect.com/science/article/abs/pii/S0049017220302912
We have doctors from Mayo Clinics to thank for prednisone. Depending on one's perspective, they can be blamed as well.

The following paragraph from the above link speaks volumes:

"Biologics have been developed and promoted that are used to replace and taper off low dose prednisone. Yet, glucocorticoids used appropriately can be the cornerstone of effective, safe, and inexpensive treatment of early active rheumatoid arthritis."

RA has come a long way. I would emphasize "early active RA" and not a long term treatment for RA.

PMR/GCA have some catching up to do.