What is the role of pressure differential switch in merv filters? is it requried interface with bms system or not?

It is because when the filter is full of dust then the pressure difference will be more so it will buzz alarm with BMS that it is time to clean the filter. 

When the filter is new the pressure drop across the filter is less but when it is dusty the pressure drop increases and this switch buzz alarm on BMS to change or clean filter when this drop is too much high.

It is good to interface with BMS so you can have the report when you need to change but not mandatory.

Hope this will work