The policy will primarily make the process of making your publications open access and complying with funder and REF requirements more simple. You will no longer need to check whether a journal is compliant before submitting, and Library staff will no longer need to check licences and embargo rules from publishers.
There are also significant financial benefits to not paying high APCs in journals which have only a gold route to funder compliance, which LSE or the funder might pay. Not only does this direct funding away from research, it is not equitable within and across institutions.