At breakfast the other day, my husband and I saw a man in a wheelchair that had angled wheels. Being engineers, we discussed the possible reasons, and decided that it was a stability feature. The chair got a wider wheelbase without making the whole chair all that much wider, and since the tops of the wheels angled in, it would be as easy to propel as a straight-wheeled chair.
We wondered secondarily if it had to do with maneuverability.
So, anybody able to enlighten us?