The motors on W123 cars are supported in the front by a pair rubber motor mounts. The mounts attach the motor to the frame of the car while buffering the motor to prevent excess vibration from being felt by the driver.
When these mounts begin to compress and soften, with age and with deterioration from exposure to fluid leaks, increased vibration will be felt in the car. In addition, the motor will ride lower in the engine compartment and, in time, will lower to the point where the accessory belts may rub against critical parts like the oil cooler lines. This can lead to a leak in the line, loss of oil, and eventually catastrophic engine failure.
So as you can see, the mounts play a critical role in the operation of a W123. Replace your worn mounts early, before trouble starts.
To reassemble your device, follow these instructions in reverse order.