Question: during the analysis of flow charts that involve computing flowrates...

During the analysis of flow charts that involve computing Flow-rates, components of streams, grams of components, mols of components, etc... I do not understand when I need to use an "Average molecular weight"

- What is Average molecular weight? (I understand how to calculate it, but conceptually I do not understand the application of it in other calculations)

- When is average molecular weight used in Chemical Engineering?

- What are the signs I would see in a problem that would indicate I should be using an average molecular weight?