Soldered leads (where the components meet the PCB {circuit board}) can only take a relatively small amount of physical weight/stress loading.
Bigger/heavier components, can exceed the safe limits, and be too heavy for normally soldered lead connections.
Without some form of extra physical connection to the PCB, the excess weight can eventually break the relatively weak soldered joints and/or metal fatigue etc the leads.
Example
Imagine it is fixed inside a car, or even travelling by car long term. The repeated stress/strain on the excessively heavy components (as the vehicle vibrates and drives over bumps in the road), can eventually break the device.
Sometimes external component fixings are used (plastic clamps etc), which screw to the PCB, and firmly hold very heavy components, e.g. very big capacitors.