there are several types of dates :
best is used before...= doesn't mean you can't use it afterwards. It's just a way to tell you the best flavor . It is NOT a safety date
Sell by = is NOT a safety date either
Use by = is a safety date unless the products are chilled immediately. This date is the last one recommended with a high peak of quality......But you may eat some products afterwards like plain yogurts.....anyway, you'll be able to taste if it still good or not.
Avoid fish, meat, not home made food, desert with cream....
All those different dates aren't helping consumers and a lot of food is wasted.
To answer more quickly to your answer it helps microorganisms, as bacteria, yeast,..Among the bacteria , some are pathogenics and may cause foodborne illness
hope I helped
