The Clarion workshops (Clarion at UCSD, Clarion West in Seattle) are key elements of the pipeline for producing excellent new science fiction and fantasy writers; I am a graduate of Clarion 92, and have taught both workshops, and volunteer on the board for The Clarion Foundation, which oversees the Clarion workshop at UCSD.
Leaving your job and family for six weeks to attend a Clarion workshop is hard enough to manage, but adding in the tuition (which is kept as low as possible, but is still substantial) can put the Clarions out of reach for many very promising writers.
So the Clarions are continuously in fundraising mode, raising money to give to promising students in scholarships to help defray tuition and other expenses. And the key to that fundraising are the annual write-a-thons.
Every year, established and new writers sign up to write a certain number of words over the course of a couple of months, and ask the people who love and support their work to pledge to donate to one of the Clarions based on the number of words they write.
I've participated every year for many years, though this year it'll be as a donor, not a writer (I just published a book, have two more in production, and just turned in yet another one, so I'm on a bit of a hiatus, though I do need to write a short story for the fall, so I just might end up on the roster after all).
And if you're a writer, consider signing up for the write-a-thon of your choice!