Stable Diffusion is the open-source foundation that most of the image-AI hobbyist and developer world is built on. Free to run on your own hardware, it gives you total control: every parameter, an enormous library of community fine-tuned models, and no content filter when self-hosted.
That freedom comes with a learning curve. The base model is unremarkable; the striking results come from the ecosystem of checkpoints, LoRAs, and node-based tools like ComfyUI. If you want to tinker, own your stack, and pay nothing per image, Stable Diffusion is unmatched. If you want to type a prompt and get a great image in one click, start elsewhere.