Because I just saved a comment draft, went looking at something else and wanted to go back. The problem was that I couldn't find the right post/comment where I saved the draft.
I thought Boost saved that because going to reply to the same comment would automatically bring up the draft.
I agree. I was completely unaware of that button and I was looking everywhere in the settings for an option to delete (accidentally) saved drafts or even clear all of them if necessary. Maybe we two are the only ones but it seems a rather unintuitive location. I understand why it's there: to use a drafted comment several times in different threads, but the feature I was looking for was to delete a draft and it didn't occur to me to look in the editor for such a feature.