A lot has been said, but to add to the list I'd say it gives them access to quite a large pool of free testers.

LLM architectures and optimization techniques change rapidly and by releasing open-weight models a lot of enthusiasts will evaluate new models for free, help implement support in inference engines, catch bugs etc. (and in turn, ofc, get a new model to run for free, so it's at least somewhat symbiotic).

We have at least seen this quite obviously when Alibaba released Qwen3-Next, which was a somewhat undertrained but still useful model which introduced the architecture that their latest models now use "in production" (also their paid "Max" models).