With tougher, more "advanced" rigid lines available, why do hydraulic systems still can't do without hose? The answer isn't in a flashy parameter but in the word "flexibility." Flexibility is not the hand-feel of "soft" but three concrete capabilities rigid lines can't give.
Eating the vibration
Hydraulic systems are never short of vibration — pump pulsation, engine running, breaking and drilling impacts. If these slam straight into valves, fittings and tanks, the accumulation is loosening, fatigue and leakage. A hose's flexible body is like a "cushion," digesting much of the vibration right in the line and protecting the more precise, more vibration-sensitive parts behind it. Swap in rigid lines and the vibration has nowhere to go but to transmit, hard.
Letting the system "yield," absorbing displacement and error
Machines expand with heat, parts move relative to each other, and assembly always carries tolerance. Rigid lines demand "straight and aligned"; a little off and they hold stress. A hose can "yield" — bend when it should, give when it should — absorbing that displacement, error and thermal deformation calmly, so it never becomes extra pull on the fitting. Many "fitting keeps leaking" problems trace back to a missing length of flexibility.
Moving with the machine, without cracking or leaking
Excavator booms swing, robot arms extend, follow lines must move with the equipment. This "pressurized while moving" job defeats rigid connections; only a hose can hold pressure while bending and swinging repeatedly without cracking or leaking. Here flexibility is not a comfort option but the precondition for moving parts to stay connected and stay connected for long.
A hose's first core skill is its willingness to be "soft." That softness is not compromise but taking on vibration, displacement and motion for the whole machine.— the hose body gives way so the system endures. That is why hose has existed for a century, and why even advanced rigid schemes can't sidestep it.