Meta's primary business is capturing attention and selling some of that attention to advertisers. They do this by distributing content to users in a way that maximizes attention. Content is a complement to their content distribution system.
LLMs, along with image and video generation models, are generators of very dynamic, engaging and personalised content.
If Open AI or anyone else wins a monopoly there it could be terrible for Meta's business. Commoditizing it with Llama, and at the same time building internal capability and a community for their LLMs, was solid strategy from Meta.
I guess it lets them better utilise their hardware in quiet times throughout the day. It's interesting they all picked 50% discount.