We could turn the question around: Why remove any restrictions at all? The answer is to share knowledge and accelerate research. Barrier-free access helps readers find and retrieve the research they need, and helps authors reach readers who can apply, cite and build on their work. Knowledge has always been a “public good” in the theoretical sense that consumption doesn’t deplete it (it’s “nonrivalrous”) and consumption is available to all (it’s “nonexcludable”). OA makes knowledge a public good in practice.