Vinegar in the rinse also helps
Laundry sours such as white vinegar only are of aid if one laundered with alkaline pH substances such as soap, washing soda, etc... OTOH if one has used neutral to near pH there is less of a clear benefit.
The whole idea of laundry sours is to bring down the often high pH left from laundering processes. That and to combat the residue (mostly sodium bicarbonate) left from using carbonates and other alkaline substances. Bringing down the pH leaves textiles feeling less harsh and removing alkaline residues helps prevent browning and or scorching when laundry comes into contact with heat (dryers and or ironers).
However you have heard me use this word before; encrustation. That is the process whereby hard water minerals, detergents, soaps, soils, etc.. become trapped in textiles when high pH are used for the wash.
All alkaline substances cause natural fibers to swell, this is why chemical processes to change the color and or textile of hair (straighten or curl) often are alkaline. Problem is when it comes to laundry as the rinsing process begins those open "pores" if you will begin to shut down, trapping the aforementioned substances. The solution of using acids (laundry sours) after a series of rinses works well enough, but can damage some textiles and or colors in the long run.
The latest thing is commercial laundries have moved to liquid detergents with their neutral to only slightly alkaline pH. They clean without the brute force of highly alkaline substances thus textile fibers are not forced open. No opening of textile weaves helps deal with the encrustation problem. It also simplifies the laundry chemical profile since you can get rid off some rinses and skip laundry sours.
People often say the clothing feels softer after using a liquid detergent instead of powders. Well it would wouldn't it? Because of the absence of soda and other caustic substances. Liquids also are less aggressive to colors on average than most powdered detergents. In fact for many if using a liquid detergent fabric softeners in general can be skipped.