So you're describing the situation between having an article and not in the plural. Case is used to show different parts of speech clearly. If you remember that when you have the feminine article "die", this is always followed by the "-e" ending. To create a distinction, the "-en" came about to show that it isn't feminine singular - to add more clarity in exactly what is being said. The reason it doesn't have this without an article is so that there are options to demonstrate cases like the dative and genitive clearly.
Long story: To create more distinction between cases and singular/plural
Short story: Because it is how German has developed.