Pressure gauges are used to indicate when the filters are blocked however for this a gauge is required before and after each filter. The pressure difference between them when the system is under normal or design flow indicates the pressure loss across the filter cartridge or how blocked it is. If there is low flow at the time they are checked then the pressure loss will be less when in fact the cartridges may need to be changed. Most customers know when the water flow if the house drops its time to change the filters.
They are not necessary, have no effect on the performance of the system, they tend to get damaged, most customers have trouble knowing how to read them and make sense of it so we generally don’t promote them. We do offer the option to those who particualy want it.