If you're building any kind of product/service that uses AI/LLMs the answer is the same as why any company would want to run any other kind of OSS infra/service instead of relying on some closer proprietary vendor API.

  - Costs.
  - Rate limits.
  - Privacy.
  - Security.
  - Vendor lock-in.
  - Stability/backwards-compatibility.
  - Control.
  - Etc.