I have read several articles that state that they were used by PRR specifically because they had a huge supply of anthracite coal and the wider firebox was designed to accomadate that. It turns out, the wide firebox also was good for bituminous as well. The ACL ran many of these also.
A couple of articles did allude to the "better visibility" version as well, but they all seemed to agree, the wider fire box was the main reason for the design.
Don't take any of this to the bank though. Greg