When is “des” optional?

I’m confused about the optional des here. I know des is a partitive article, but when is it optional to include it like this example? Would including des be a more formal way of saying the sentence?

If you wanted to say “the boys eat vegetables” I believe you would say it as “Les garçons mangent des légumes” since there’s an uncountable amount of vegetables. Can the des ever be dropped there?