It would make the most sense because, when reading a book, for example. You might get a sentence similar to, "Brianna started the party (Which she spent 2 hours on.)"
The first function of parentheses is to offer extra information. Parentheses communicate to readers that the material inside the parentheses is not necessary to understand the main sentence, nor is it part of the grammar of the main sentence, but is pertinent enough to be included.
