I've noticed that - while browsing through questions and answers - I regularly come across an interesting question/answer but reading and understanding the answer or answering the question would take quite some time. That's fine, but we all have other things to do as well, don't we? So it happens that I think to myself: I should remember to come back here and fully try to understand this answer or write up an answer to this question when I have more time. (or even adjust an answer following a comment) However, I rarely do.
What is possible (and I have done this before) at the moment is to favourite such questions, but I don't think that's what the 'favourite' option is there for, is it? If it is, this is mostly a suggestion to alter some of its functionality (and add it next to answers as well). If it isn't, here's what I think we could use: something like a reminder, a literal note-to-self. Perhaps a 'Revisit' button, which gives you a similar alert to when people have mentioned you in a comment or something, only that the alert doesn't go away until you have removed it yourself by unclicking the 'Revisit' button for that particular question/answer.