They matter, because they provide information. They just aren't the only thing that matters. If Biden was going into the election leading in national polling by 10%, that would be a strong indication that he was going to win the swing states that are the deciding factor.The election will be won or lost at the state level. National polls don't matter at all.
Ultimately the only thing that really matters, in an extreme, bottom-line, minimalist sense... is the final result of the election itself... but if we're if we're boiling everything down that much, I guess we can all go eff off, stop talking about it entirely and just wait to see what the election result is... and that's no fun.