Purpose of a Counter-Flange?

Counter flange is nothing but a mating flange.

you have not written at what place this question raised to you?

I assume you have seen it in your Equipment drawing , and manufacturer is providing you the counter flanges with the equipment.

If I am right then the reason is..you are procuring equipment from vendor who follows different standards than you.

so he want you to provide mating flanges with the equipment so you will not face any problem at site. (he should provide you gasket and fastners also)

If you procure pump, then some time vendor supply pumps with N2 filled in, and with blind flange. In that case vendor will provide conter flange loose (If he is providing then) and so you will need one pair of gasket and fastners more...take care....

If this not satisfies your question...then pl. give some more brief about your query.