Style Assist is mainly for articles written by journalists that are employed by third-party news organisations (who own the UK's local news outlets) but are subsidised by the BBC - these people are "local democracy reporters".

LDRS articles are made free to everyone in the scheme (including the BBC), but because the content is written primarily for their home news org the articles that end up being published by the BBC need to be modified to comply with the BBC's editorial guidelines and for style reasons.

Hence, Style Assist.

I see. Journalism is much more nuanced than I had imagined. The article I linked mentions there are 1,200 rules in the BBC News style guide!