There are limitations on the amount of water that can be discharged from handheld branches, (typically 1000 lpm), as higher flows have very high back thrust, which makes it impossible for firefighters to handle them safely. Due to this, handline nozzles/ branches have limited range, and hence limited effectiveness.
To fight bigger fires, which give out large amount of radiation, it is necessary to throw water/ foam in sufficient quantity from a safe distance. As explained above, it is impossible to do this using handline branches. Fixed monitors provide the solution as they can project water/ foam further due to the higher quantity of agent, and their design and installation allows the back thrust to be handled safely.