In theory, you wouldn't need slotted dropouts on road bikes, and in fact they ultimately wear away in favor of vertical dropouts.

But the level or manufacturing precision that makes vertical dropouts possible wasn't that common BTID, so the slotted design created a greater margin of error. The design is also a carryover from pre-derailleur frame design, where a slot was needed to set the chain tension, and the wheel was centered accordingly.

The built in stop on one side, is there to optimize the wheel position for the derailleur, yet leaves the ability to center the wheel as was done in the past. This is functionally the same as with a claw mounted derailleur, and an alternative to the micro-adjust screws used on costlier bikes.

The fact is that absent the micro-adjusters, and some aftermarket stops, SOP was to locate and center the wheel each time it was installed, which isn't a burden except for racers for whom time is critical.