Rough-in is the critical stage when everything is still visible and malleable and nothing is permanent yet. The pipes are in place, but the walls are still open, the floor is still exposed and accessible, and mistakes can still be corrected and fixed before surfaces close over them. Once that window of opportunity closes and walls are sealed, changing a rough-in position or discovering an error means opening a wall or floor later, which is far more expensive and disruptive than getting it right the first time.
The fixture specification and dimensions have to be in hand before any rough-in work starts. The pipes are positioned against the actual fitting that will eventually sit on and connect to them — not to a generic or assumed distance, but to that specific model's rough-in dimension. The supply and waste lines are placed at the exact heights and distances that the chosen fixture demands. That specification is only reliable if the fixture has been chosen, purchased, and its dimensions verified against the physical product. A generic or assumed rough-in is the recipe for discovering at the finish stage that the fixture which was actually ordered does not sit properly on or connect to what was installed in the rough.
Testing before closing is the last moment to verify that everything works as intended. The supply lines are pressurized and checked thoroughly for leaks. The waste line drains freely and quickly. The positions are right and accessible. Once that test passes and nothing leaks or clogs, the photograph with dimensions becomes the permanent record of what is where and how deep, so future work can be planned around it. That photographic record protects everyone — the homeowner knows what is buried behind the walls, and future trades can see exactly where things are and coordinate their own work.