In Old Time music, most standards are by convention usually played in particular tunings. It doesn’t mean they have to be played in certain keys, just that they usually are. This simplifies things for players jamming, for instance. If the tune is going to be Cripple Creek, for instance, the clawhammer players will normally tune to A, but if it’s going to be Angeline the Baker, you’ll see them tuning again to D. There are also banjo tunings for those “mountain modal” tunes as well, like Shady Grove. There are also a number of specialty tunings that are used for a small number of tunes, like “Cumberland Gap tuning” for instance. Sometimes at jams, players will play a few tunes in a single key and then switch it up so that everybody isn’t always tuning.

Here are Wayne Shrubsall and Bruce Thomson at the Albuquerque Folk Festival, June 21, 2008 (I found the video on the Youtube machine) doing a medley of D tunes.