To me it's reading from different perspectives and other's writing style. It's really important when it comes to political stuff or anything else that you need to base a decision on.
But I think it applies even just to plain articles about a niche you follow.
Like, maybe some new GPU features get released for Linux or something. Gamingonlinux might cover it from a Gaming perspective whereas Phoronix might focus more on the hardware aspect of things and the original blog post or changelog or whatever might just give a mostly a list of changes and a description.
You don't need to use your dozens of accounts to do that. Even if you create communities on these instances with those dozens of accounts, you can still opt to use a single account to actually contribute instead of looking like a spam network. Why not do that? And no, it's not "promoting" a small instance to make posts to other instances with them. I again would argue that most Lemmy users don't pay attention to which instance an OP is signed up to.