Simply put, dirty filters can potentially not only decrease the efficiency of your A/C up to %20, but also increase your electricity bill considerably.
A dirty or clogged filter blocks the normal air flow on the indoor unit coil. Then, you can get lower air flow in compared to the normal condition. As a result, your poor A/C has to work more hours to bump down the ambient temperature to the desired set point which it means lower efficiency or higher electricity consumption.
On the other hand and in most cases, the blocked drains are the result of dirty filters. The effect of a blocked drain is damaging the walls or ceilings.
Technically speaking, dirty filters cause liquid refrigerant flood back to the compressor which is not really good for the system.
In ducted systems, you can find the filter/s in the return air duct. In such systems, filters can be generally two types, disposable or reusable. Reusable filters in both ducted air conditions or split air conditions should be washed normally every four months.