Finer grained control over the tools the LLM is supposed to use. The 'tool_choice' should allow giving a list of tools to choose. The point is that the list of all available tools is needed to interpret the past tool calls - so you cannot use it to also limit the LLM choice at a particular step. See also: https://zzbbyy.substack.com/p/two-roles-of-tool-schemas
Control over how many tool calls can go in one request. For stateful tools multiple tool calls in one request leads to confusion.
By the way - is anyone working with stateful tools? Often they seem very natural and you would think that the LLM at training should encounter lots of stateful interactions and be skilled in using them. But there aren't many examples and the libraries are not really geared towards that.