Why not just an LLM and a spreadsheet
A guess dressed as an answer is still a guess.
An LLM on its own has no engine to compute against. Ask it what a price rise or a route change would do to net margin and it will answer fluently, but the figure is invented, because nothing underneath it knows how cost actually moves through the business. Every what-if is a guess dressed as an answer, and nothing you would put in front of a board.
CostCtrl gives it an engine. The agent's job is to search: propose a change, check the constraints, ask what happens. The engine's job is to be right: every consequence computed by the same deterministic rules, every figure traceable to its source. The team's job is to make sure the model is worth searching. AI removes the friction. The methodology and the team deliver the result.