Rara Avis I cant say about Joan Baez, but I know a lot of musicians who check their tuning between songs, and others who vary their tuning from song to song.Possibly standard tuning for the first,then
to dropped D, then double dropped D. then DADGAD then back again.
    For the same reason a lot of guitarists carry two or more guitars in different tuning to reduce the possibility of breaking strings while tuning.