There are benefits to both the authors and the readers, through the removal of price and permission (copyright) barriers:
More readers & citations There is growing evidence that Open Access material is more likely to be found, read and cited than work available only to those who subscribe (see this Times Higher Education article). More evidence is provided through SPARC Europe that now maintains the Open Access Citation Advantage Service, a list of studies gathering information on whether Open Access articles benefit from a citation advantage or not. According to SPARC Europe, 65.71% of studies documented benefited from a citation advantage owing to being Open Access.
More locations If your work is included in Open Access repositories (e.g. Middlesex University Research Repository), then it will be indexed and visible to different audiences (e.g. through BASE), with a copy always available online, even in the event that the published version is not available for any reason (e.g. one publisher merges with another and experiences temporary technical difficulties). Note that reputable repositories will always point to the version of record on the publisher website, at the same time as providing an Open Access version. (See Which version can I make Open Access?)
Saves you time and effort - If your work is published without restrictions preventing it, then copies might be added to OA repositories on your behalf, bringing you new audiences, better indexing and providing you with a back-up, publicly available copy at no cost to yourself.
Benefits further research – Independent researchers and those working in small institutes and developing countries will be able to read content that previously would have been hidden behind payment barriers.
Where copyright and licensing barriers are also removed, then data mining and other computerised research techniques become possible. By choosing from available CC-BY licenses, you can choose how your research is re-used, distributed and built upon to further innovation.
More information on Open Access can be found at the Open Access Directory, which is a collection of OA-related lists.