It was helpful on the occasional reference to japanese-only knowledge, such as comedians and tv stars.
Eh, I don't feel like it really enhances it knowing what they reference. Like, the notes can say: "Note: This joke refers to famous Japanese boxer yadda yadda", but I have no connection to this person so that still means nothing to me. It's more necessary for untranslatable puns I guess.
I don't have anything against notes, though! They can be helpful, I just think Gintama's are excessive.