Different nozzles do different jobs: a penetrating nozzle for breaking through a solid obstruction, a flushing nozzle for silt and debris, a rotating nozzle for fat and scale on the pipe wall, and a chain-flail or root-cutting head for hard deposits and root masses.
Pressure is matched to the pipe. Old clay and pitch fibre pipework is jetted at lower pressures than modern PVC; a competent engineer surveys or at least assesses the pipe before selecting settings.
Jetting is the only clearing method that leaves the pipe genuinely clean, which is why it is also the required preparation step before any lining repair.