That extra $15 a month is for access to LLMs. They currently support the Claudes, the GPTs (not o1), Mistral, Gemini, Llamma, Qwen QWQ, Nova, and DeepSeek. It's currently unlimited access in the standard chat format.
You can also choose if you want the chat to RAG search results into the context for additional info, and then cite those sources. To me, replacing a Claude/ChatGPT subscription with $15 on top of a company I already like, while also getting a bunch of other models was a no-brainer.