From Peter Suber:
Authors: OA gives them a worldwide audience larger than that of any subscription-based journal, no matter how prestigious or popular, and demonstrably increases the visibility and impact of their work.
Readers: OA gives them barrier-free access to the literature they need for their research, unconstrained by the budgets of the libraries where they may have access privileges. OA increases reader reach and retrieval power. OA also gives barrier-free access to the software they use in their research. Free online literature is free online data for software that facilitates full-text searching, indexing, mining, summarizing, translating, querying, linking, recommending, alerting, "mash-ups" and other forms of processing and analysis.
Teachers and students: OA puts rich and poor on an equal footing for these key resources and eliminates the need for payments or permissions to reproduce and distribute content.
Libraries: OA solves the pricing crisis for scholarly journals. It also solves what I've called the permission crisis. OA also serves library interests in other, indirect ways. Librarians want to help users find the information they need, regardless of the budget-enforced limits on the library's own collection. Academic librarians want to help faculty increase their audience and impact, and help the university raise its research profile.
Universities: OA increases the visibility of their faculty and research, reduces their expenses for journals, and advances their mission to share knowledge.
Journals and publishers: OA makes their articles more visible, discoverable, retrievable, and useful. If a journal is OA, then it can use this superior visibility to attract submissions and advertising, not to mention readers and citations. If a subscription-based journal provides OA to some of its content (e.g. selected articles in each issue, all back issues after a certain period, etc.), then it can use its increased visibility to attract all the same benefits plus subscriptions. If a journal permits OA through postprint archiving, then it has an edge in attracting authors over journals that do not permit postprint archiving. Of course subscription-based journals and their publishers have countervailing interests as well and often resist or oppose OA. But it oversimplifies the situation to think that all their interests pull against OA.
Funding agencies: OA increases the return on their investment in research, making the results of the funded research more widely available, more discoverable, more retrievable, and more useful. When funding agencies disburse public funds, OA helps in a second way as well, by providing fundamental fairness to taxpayers or public access to the results of publicly-funded research.
Governments: As funders of research, governments benefit from OA in all the ways that funding agencies do (see previous entry). OA also promotes democracy by sharing non-classified government information as widely as possible.
Citizens: OA gives them access to peer-reviewed research, most of which is unavailable in public libraries, and gives them access to the research for which they have already paid through their taxes. But even those with no interest in reading this literature for themselves will benefit indirectly because researchers will benefit directly. OA accelerates not only research but the translation of research into new medicines, useful technologies, solved problems, and informed decisions that benefit everyone.
Are you thinking of publishing your book or journal as OA? Here are a few encouraging reasons to do so, from Why Open Research? :
Increase your visibility. Build a name for yourself. Share your work and make it more visible.
Reduce publishing costs. Open publishing can cost the same or less than traditional publishing.
Take back control. Know your rights. Keep your rights. Decide how your work is used.
Get more funding. Meet funder requirements, and qualify for special funds.
Publish where you want. Publish in the journal of your choice and archive an open copy.
Get that promotion.Open access is increasingly recognized in promotion and tenure.