A company like the BBC can post content to their own website completely under their own control. They can build their own platform that completely controls commenting. They can pay a large staff to build and maintain that platform exactly as they see fit. So why do they need Twitter?

Seriously, what is the value of Twitter for them? It isn't the same as for the masses who need somebody else to build and maintain all that infrastructure. For entities capable of running their own networks, Twitter and TikTok and all the other "social media" serve a different function. Indeed, when you are not only capable of running your own datacenters but actually do run them and operate your own media network, but still include Instagram as a core component of your business strategy, it is because it provides a value you aren't realizing from everything within your own datacenter.