What happens as the code changes? Why is this better than the agent using grep and LSP in a targetted way for the task at hand, against the current code, includ

What happens as the code changes? Why is this better than the agent using grep and LSP in a targetted way for the task at hand, against the current code, including the current diff? How does one get an agent to prefer this tool to the ones it's been trained on and uses almost immediately? (ignoring instructions to use alternative code understanding tools, I've tried getting them to use LSP, but they all most always ignore those instructions)