Building width is relevant in every load calculation regarding the ability of a girder or beam to support the imposed load.
The reason is that the wider the building, the longer the joists and rafters.
The longer the joists and rafters, the greater the dead and live loads placed upon them.
Since these increased loads will terminate on the girder, the girder must be properly sized according to the building width or the length of the joist and floor, rafter and roof loads that may be imposed upon it.
Here is an example from the engineering tables:
A girder that is made of 2 2x12s supporting only 1 floor with a building with of 20 feet can only span
That same 2 2×12 girder can only span 7′ if the building width is increased to 28 feet.
Because in the first case the length of the floor joists are only 10 feet long coming from each side and terminating on the middle girder.
In the second case, the joists are now 14 feet long.
The added weight from the joists and sheathing and loads from added furniture and human loads no longer permits the 2 2x12s to span 8’1″…and the span must be reduced to 7′, or the girder must be upsized.