Tool Use in Autogen

I created a tool to be used accorsdint to the documentation on Autogen.
But I am confused or it is not clear to me that when we register our function or tool with the LLM agent why not it calls and executes it . why creating another NOn LLM agent for execution. also the Non LLM initiates the conversation but also performs the tool.
why not LLM powered agent perform this task itself.