By increasing the visibility and accessibility of your research outputs you enable more people to engage with your research. With more engagement, greater impact may be achieved.
"What Open Access does is to maximise audience size so that articles that are worthy of citing
stand the maximum chance of being seen by anyone who might have reason to cite them."
Swan, Alma (2012) Policy Guidelines for the Development and Promotion of Open Access, UNESCO, p.29.
"There is an interesting relationship between altmetrics and open access. One could even refer to altmetrics as open metrics. This is firstly due to the fact that altmetrics data uses open sources. Altmetrics services access and aggregate the impact of a research artefact, normally via an application programming interface (API) made available by the source. Altmetrics services in turn provide APIs for embedding altmetrics into institutional repositories or third-party systems. Secondly, open access research outputs that are themselves promoted via social web applications enjoy higher visibility and accessibility than those published within the commercial scholarly communication model, increasing the prospect of public consumption and engagement.
Altmetrics (also known as article level metrics or ALMs) are seen as complementary to open access."