Most people think of suspension as comfort. It is not. It is the set of parts that keeps each tyre pressed onto the road, and every other system on the car depends on that. The brakes can only slow a tyre that is gripping. The steering can only point a wheel that is held straight. A tyre can only last if the wheel it is on sits square.
Suspension wears slowly, which is the problem. Shock absorbers lose their control a little at a time over years, so you never feel the day they went. A bush perishes, a joint loosens, and the car quietly gets worse at stopping and starts eating its tyres. The only sudden ones are a broken spring, which you hear, and a failed joint, which is rare and which we take very seriously.
The other thing worth saying is that the parts are not expensive on most cars. What gets expensive is what worn suspension does to the tyres, the brakes and the MOT while it is left. A knock is a cheap warning. Take it.
We lever every joint before we name the part. The knock you can hear is often not the only thing that is worn.