If you are a researcher looking for journal articles you may choose to access Open Access material in preference to locating copies of paid for material via your library or paying a fee to the publisher. There is an argument that Open Access increases the use of research because it is freely available. A counter argument would be that researchers should choose the material that is relevant, however, they obtain it. The truth is probably that researchers make judgements on a number of criteria including immediate access to the full text.
You may also choose to search for only Open Access material. Searching in institutional/subject repositories you are more likely to find Open Access material. Some search engines offer an Open Access filter. Librarians would advise searching for all available resources and make the decision on what to access in full text on the basis of relevance to your research.
If you are an author you can consider publishing in an Open Access journal. If your research is paid for by the government or by a research council or body it may be a requirement of your grant that you publish your research in an Open Access form. In either case there will be a cost of between 1000 - 3000 GBP to be paid to the publisher. Normally this would have been costed into a grant application, individual authors may have to secure additional funding to pay publication fees.