One of the things I used to be required to do when I pitched a proposal for a new book to my usual publisher was write the blurb that would appear on its back cover. The justification was that if I couldn't compose a two-paragraph summary of the book, I didn't have a good enough idea of what I was proposing to actually write it.
The same concept applies to the fanfic writer. Regardless of how you release your story for reading, you're almost always going to want or need a summary. And writing that summary before you finish (or begin!) the story is a good test of how well you've planned out your story.
If you can't boil down at least the start of your fic to five or six sentences, you probably don't have a good enough idea of what you're doing or where you're going. Which is to say, you've bobbled the critical development stages of writing that lead to a good story. You have to stop what you're doing and put in that work, because you're going to need it.
This of course assumes you're not just too lazy to bother with a summary. If you are, well, remember that a good summary will pull in readers -- think of it as advertising. If you want people to read your work, it helps if you make the effort to "sell" it.
And if despite that you're still too lazy to bother, well, you're probably too lazy to care about writing well. So maybe it's better if you don't. That way the number of disappointed readers will be smaller.
