And, on older steam locomotives, with out a screw feed, even with a slope sheet in the tender,someone had to shovel the coal from the rear of the tender to the front, where the fireman could get to it quickly.
Easier to have the third man riding back where he was needed, instead of the cab, where he was in the way.
Note that when steam powered stokers and coal feed screws came into use, the doghouse went away.
Ed