I think (because the BBC has not said explicitly) that it's partly to do with decreasing the cost to the BBC of licensing content for which they do not own the IPR by restricting those licences (i.e. numbers) to UK TV* licence payers. But mainly, I think, it's to increase income by licensing content for which they do own the IPR to other broadcasters (or narrow-casters). Totally understandable, IMO, in their current financial and political situation. It's what they've done in television for years - but UK television was never broadcast beyond the UK.

* There is only a TV licence in the UK, which also pays for BBC radio. There used to be a radio licence, but it was abolished decades ago.