Dual axis in Tableau is essentially used for comparisons. Basically to compare measures in Tableau.
A measure in Tableau is a field that is independent; meaning it is self-descriptive like date, name etc.
For e.g: If you want to visualize both shipping cost and profit across time in the same graph, you can use a dual axis graph instead of going for two separate graphs.
And this is what it will look like:
If you want to learn more, follow these blogs:
What is Tableau | Data Visualization Using Tableau | Edureka
Tableau Tutorial | Step by Step Guide to Learn Tableau | Edureka