Theyre also a good indicator on where the plant is at that moment. Late in flower, if they didnt already, many of the bottom leaves will dry and fall off on their own if you dont remove them. I use that fading in the end to make sure my substrate is clean of nutrients and they start using their reserves for their last push. I grow organic so its not really necessary to flush (debatable but some studies suggest that) but most people seem to agree that the buds taste better if you let it fade off. I like to keep them green as much as possible in the meantime tho. If you think about it, it makes sense. You cut your plants deep in the dark period or after 2-3 days of darknes so it regenerates terpenes for that ammount of time, but also because they send the chemicals they use for photosintesis to the roots when its dark time, leaving you with buds with less chemicals in them (be it organic or not, theyre still chemicals and will likely affect the flavor but at least not like sinthetics which gives them a bitter taste if you dont flush properly, also more nocive to smoke)
EDIT: And during the grow they will also tell you if they lack nitrogen, I call those safety leaves. When I start seeing fading on them I up the nutes and I avoid for upper more useful leaves to suffer the consequences. Youre more likely to cause less harm by using too much PK also since the little damage you can do (unless youre an animal) will be divided by more laves, more leaves to absorve the extra PK means less leaves carrying too much or at least less excess ammount in them.