Diesel engines don't burn fuel by ignition (ie. spark plug). On the intake stroke, the piston moves down and draws air only. When the piston moves up, it compresses the air, thereby raising its temperature. Then fuel is injected and consequently burned by the hot air, producing power.

Kapag malamig, malamig din ang cylinder. So this has the effect of reducing the temperature of the compressed air, which might result in hard starts. Thus, heater plugs are used to compensate.

Hope this helps!