Dunno about the bleach part, that might be in some as well, but typically white fabric detergent contains optical brightener that counters the typical yellow tint of worn garments by emitting extra blue light (and your eyes perceive the full presence of the spectrum as white). That’s also why this whitening effect will fade off if you then use detergent that doesn’t contain brighteners: you are washing out those blue light particles once again.
optical brightener
AKA blue dye.
The process is also called “bluing” and existed way before they made up a scientific “you have to buy this product, you can’t do this at home” name.
Well that unlocked a memory. I was on a road trip around California and stopped off in a small town to do my laundry. An elderly gent was already in the laundromat and the washing machine window showed bright, bright blue. He said he recalled that his late wife used to use blueing tablets to get the sheets etc white. “I couldn’t find any at the store, but these toilet cleaning tablets are blue, so figured I’d try them.”
This is what my late mother used: https://www.retonthenet.co.uk/vintage-washing-laundry-reckitts-bag-blue-reckitt--coleman-hull-dolly-bag-1960s-nos-dolly-blue-5487-p.asp
Optical brighteners aren’t the same thing as blue dye, even though they have the same role. Optical brighteners absorb high wavelength (UV) light and re-emit it as blue light. This helps prevent the new particles from overpowering or covering the existing dye. Blue dye would just paint the clothing more blue, which usually isn’t what you want.
So in this case it isn’t just marketing bla bla for some old thing, it’s actually a new(er) thing that replaced the old thing.
Honestly I’m more in the “buy durable fabrics and treat them well but if they acquire a tint or lose color over time so what” camp. Good linen shirts for instance will still look great after a long time, never mind any fading. For some stuff it can even enhance the optics like the famed worn out jeans look.
It’s really the whites you have to worry about acquiring a tint over time. It makes them appear dingy, aged, even yellowed, so you may need to replace them sooner. Bluing or brightened will keep whites brighter for longer, so you can keep them longer
