The danger is cross contamination. If something goes bad in one of them. Anything from using tainted meat to you mishandling it in one way or another, you’ll be carrying that over to the next pot. Dumping the stew and cleaning the pot/utensils is a good way to stop that from happening.
If you go to a restaurant that serves soup they’ll usually have the same soup on from one day to the next if they have a good amount left over or maybe the have one or two soups that they always have, but no matter what they do almost every restaurant will, at some point, dump their soups so they have a chance to wash the soup kettle and ladle. That way if there is something in there that’s making people sick, they don’t just keep doing it forever (or until they hear about it), it gets stopped at some point.
Now, I know, most bacteria/germs/pathogens should get killed above a certain temperature, but most homeowners don’t know these temperatures, don’t temp their foods and don’t move them through the danger zone in less then two hours…which, for a stew, depending on the size of the batch may require an ice wand or, at the very least stirring every 10-15 minutes if it’s in the fridge.
@AmateurBarbarian. It’s true, you only need to get beef (I’m assuming beef stew) to 145, but then you have to get it from 135 to 41 in less then two hours. Then, when re-heating, you have to get it from 41-135 in less then two hours, THEN you have to get it to 165 and maintain it over 145.
That danger zone thing is what’s important, it’s in that range that the bacteria can grow the fastest. So, if something gets in there and you cool it too slowly, the bacteria can multiply very quickly. Then you reheat some of it in a microwave the next day to, say 130, and you end up sick.