Timeliness: by archiving a preprint, I am able to reach potential readers in a much more timely fashion.
Impact: by sharing the article openly, more people are reading it, and acting on the ideas. Others have demonstrated the academic impact advantage of open access (increased citations) through research. This is real world impact - most likely to be relevant for faculty in the professional programs. This makes professional practice informed by research possible, illustrated by the trend towards evidence-based medicine.
Access: people whose libraries do not have subscriptions are much more likely to read the article.
Prestige: for those who do write and present a fair bit, this is a way to show off. This is not (entirely) self-serving; read on...
The description of her blog also gives you an idea of her perspective on all this:
Imagine a world where anyone can instantly access all of the world's scholarly knowledge - as profound a change as the invention of the printing press. Technically, this is within reach. All that is needed is a little imagination, to reconsider the economics of scholarly communications from a poetic viewpoint.