You have a single set of guidelines (BBC doesn’t have two sets, nor does any other provider I know of) and a single set of subtitles as a result.
This is accessibility like anything else. High contrast can be used by many people on the ability spectrum for multiple various reasons. Subtitles are the same. Some use them because they’re deaf, some use them because they’re not confident enough in a language to use only one sensory input, some just like subtitles.
But since your claim is they’re separate, please show me exactly how they are separate. Show me the two separate style guides, show me providers differentiating between them and providing both captions and subtitles. Show me even the smallest sliver that backs up your claim. And, more importantly, show me the exact reasoning why color would work in one but not the other.
Similarly, subtitles are necessary for on-screen foreign language text (e.g., signs or documents), but not when the text is in the language that a deaf viewer understands. In other words, you wouldn’t have an English subtitle for an on-screen sign in English, but you would for a sign in Chinese.
As the BBC produces content exclusively (or nearly exclusively) in English, these guidelines are surely directed towards captioning for hearing-impaired audiences.
As to why: it’s another datapoint to use to reconstruct what fully able people can do quickly. Sometimes captions aren’t well timed. Sometimes a lot of things happen very quickly and it’s hard to keep track. What if the camera isn’t on anyone’s face? You can probably figure it out by context, but why expend the mental energy when it can just be colored?
Just because you never had this issue (and I’m sure you did, it’s not like we remember the most mundane of details like this years later) doesn’t mean it’s not an issue.