Inclusiveness. The miniscule minority of people who can understand British Sign Language versus the majority of people who can read English subtitles. If a Venn diagram was drawn there'd be a big circle for the people who can read and inside it a dot representing the people who understand BSL.
I doubt there is *anybody* who understands BSL who cannot read subtitles (we're not talking of junior school children here).
What about all the people who are hard of hearing or maybe are watching this in a noisy environment who need to see the subtitles? That would be inclusivity.
This is pure virtue signalling.