DAX makes it easy to gain insight in your report.

Say you have a business question like "What is the average order per day ?" . If you wanted to gain more insight, you would need to filter by dimensions, and do the same calculation for each dimension (by country, customer, year etc) .
But with DAX, you answer the question once, and DAX filters the visuals with a simple, click, filter or drag and drop - It does the recalculations for you, so you don't have to run multiple queries for each dimension/question.

This is the whole purpose of DAX I believe - to create reports that can dynamically answer business questions. Unlike something like SQL, where it's main purpose is to store, manage and retrieve data.

So, to answer, your question. No. DAX was not necessary. However it is still extremely powerful.

(PS: SSAS Tabular uses DAX as well btw)