The central function of Photometry is accurate measurement of an object, typically a star, in terms of its brightness either overall magnitude/flux by dividing the flux into varying bands of the Electromagnetic Spectrum.
Whatever your location is on the Earth, you use a defined measurement system to determine the length of varying objects: Imperial and Metric.
When measuring stellar flux, what do you perform measurements and analysis against? This becomes the ultimate question so that there aren’t multiple different measurements for a singular object.
Standard Photometric Stars provide a means to, not only calibrate your photometric system, but give a common “yardstick” from which all measurements can be compared.
IMPORTANT: This focuses on standard stars, not standard filter types.