Bakers proof their breads twice because a longer proofing time adds additional flavour. You can’t simply combine the time into one extended rise because the bread will expand way too much (overproof), lose elasticity, and collapse in the oven. The second rise is also supposed to lighten up the texture of the baked bread, although I haven’t noticed much of a difference myself. Surprisingly, however, I’ve found that you can get away with a single rise, just keep it the same length of time as you would a normal bulk ferment. See results of Experiment 24 for a more in-depth discussion.