Open access means making your research outputs freely available online. The aim is for anyone anywhere in the world to be able to read the research/publication and know what purpose they can use it for.
Using open access, authors can extend the reach of their research, allowing readers access to material which would otherwise be behind subscription paywalls or other restricted systems.
Whatever stage you are at in your research career, making your work available to the widest audience is going to extend the reach and impact of the research.
Making work freely available can increase citations and increase the potential for collaborative research. Internet search engines such as Google Scholar search Open Access material deposited in institutional repositories, increasing the visibility of your work.
Open Access compliance is an essential requirement from research funders in the UK, including the UKRI. If you are an author of an output of research that has been in receipt of external research funding you need to check and ensure you meet their open access requirements.
What is more, while open access is certainly about more than the Research Excellence Framework (REF) non-compliant items are not eligible to be submitted.