worst path vs. equivalent pressure drop

[mdt22]

I have a fundamental question:
When sizing a pump or fan we typically add up the complete pressure drop for the worst path and directly use that for the pump or fan pressure required.

So what is equivalent pressure drop used for? Say you have 3 parallel paths of 3in-wg each. The equivalent pressure drop is only 1in-wg when the 3 parallel paths are combined into 1 equivalent path. This webpage discusses equivalent resistance:

http://www.pmengineer.com/CDA/Archives/b8975647d7298010VgnVCM100000f932a8c0____