In most parts of the United States indeed the world tap water is or near neutral to slightly acidic. This of course will depend upon source of the water and or how it is treated (if at all) before being piped into homes.
Base (alkaline) substances enhance cleaning as they react with fats/oils/grease to form (for lack of a better word) "soap". This is why soaking laundry in an alkaline bath (washing soda, borax, TSP whatever) loosened soils and helped break down oil and grease.
Most all soaps are alkaline in solution (which is one of the ways soap cleans)but some are near neutral. Soaps in general due to their pH and being very good at dissloving oils tend to by drying on many people's skin. There are ways around this but we're not on that right now.
Detergents based upon chemcials can be neutral to highly alkaline depending upon what types of builders,surfactants, and other chemicals are used.
Buffering agents are added to not only protect textiles, but washing machine parts and certain other chemical components of the detergent complex such as bleaches, enzymes and so froth. This why many vintage and even modern detergent packets advised persons not to add packaged water softeners and or other chemicals (aside from perhaps LCB if the detergent didn't contain bleaching agents already), but simply add more product if the load was grossly filthy. The goal is not to interfere with the balance so each part of the detergent can act as it's supposed to.
Because previous laundry detergents including soap contained were alkaline, sometimes extremely so, there was a need for a sour (acid) rinse was required to remove by-products of base substances (mainly sodium bicarbonate)and to ensure textiles were left near or at neutral.
As one has stated previously multiple rinsing with water and testing the rinse baths is often all that is required to bring a high pH wash load dwon to neutral. However this could mean lots of rinsing and again depending upon the quality of local water supplies one may never get there, hence the laundry sours.
There is often a mistaken idea that laundry sours "remove" detergent and or soap residue; that is not the case.
Acids counteract bases, the laundry sour merely deals with any remaining base substances such as the aforementioned sodium bicarbonate. If there is soap residue the acid sour will bind with the fats from the soap to create fatty acids. This residue if not removed will rot and begin to give off a whiff in textiles. It also will attract dirt and oils so items will soil sooner. Finally it is one of the sources of tattle-tale grey laundry.
If detergents (petrol based) were used for the wash and the base residue is greater than the amount of sour being added instead of cancelling out the former will overcome the latter. Commercial laundries use various methods to test the final pH of a rinse bath and or sour bath to see if things are where they need to be. Commercial laundry rinses will give dosages designed to deal with a range of pH as well.