This boy also purred for a full day and near constantly for the days that followed when I brought him home. When I looked into it I learned that cats also purr to themselves in emotional distress, as a way to comfort themselves.

Obviously it wasn't to deal with fear as he confidently explored the new home, but being taken from the shelter by a new person into a new place surely has to leave a cat unsure and nervous about what's happening in their world.