An Author Jefferson asked basically this question this May in his blog, which I just read. There are some authors whose blogs are posted pretty much the same day as their new chapters that give details on what happens in their newly posted chapter.
I took a stab at giving a reason, but I suspect I am wrong: I think saying at least part of what is happening in the latest chapter posted is "traditional". Lots of (well a few) authors do it. It is probably intended to attract readership. Maybe it keeps a continuing story in a potential or past reader's mind, even though he didn't have time to read this weeks (months, semesters, years) installment. Its an investment in the future. If a reader keeps his eye on what is happening in the story, maybe he will pick up and read next time since he won't be disoriented by not knowing what happened last episode. I know, its kind of lame, but at least its a reason.
On the other hand, instead of reading the story you can get the Cliff Notes version by reading the author's blog, either every time it comes out, or saving all the blogs for a rainy day and reading them once the story is done.
Like reading the actual story at 2,000 words per minute. Like reading a fairy story "Once upon a time...And they lived happily ever after."
Ok, now you have "read" (for some values of read) every fairytale ever written. Except the ones in a foreign language.
All purpose SOL story: Introduction, character (repeat characters as needed) plot, sex, sex, sex. The hero wins. The end. Details of the plot vary. Sometimes there is more or less sex. On Fine stories they leave it out almost entirely. Sometimes married women have babies, it must be immaculate conception.
You probably can tell I am not a writer. I do occasionally review stories. I am not very good at that, either.