To add another part of it, for people using BSL, it's akin to their mother tongue.
Being able to watch content with signing is akin to having a TV show dubbed into your native language, rather than relying on subtitles.

Edit: I just had a check, and it's actually mentioned in the ofcom guidelines:
Subtitle users reflect the full range of proficiency in English; some profoundly deaf people regard BSL as their first language, and are less fluent in English.