This is not a thing that they do, at least no more than anything else in the Star Wars universe. The scene literally establishes the ship having internal gravity as a plot point. The bombs would drop just as readily as the bombardier and the detonator did. when they fell to the bottom of the bomb bay.
True, & when the bombs left the artificial gravity envelope of the bomber, they'd continue along at the same velocity they obtained dropping from the bomb bay. For that matter, the gravity at the bottom of the bomb bay could be sufficiently stronger than the rest of the craft to impart extra velocity when leaving the bay. As for why would they do this, IMHO for the same reason F-16s today have a bomb impact calculator on the HUD. Sometimes you don't have, or don't want to use, smart weapons for your job.
That being said, I think a WWII analog of a torpedo bomber would have been more cinematically interesting. Call it a ST-5, 3 person crew (1 pilot, 2 gunners), & replay a sort of "Attack at Midway" type thing, & it might have been a bit more exciting...
Damon.