Hi Sivarma,
Quick intro: variables store mostly expressions (like sum(Sales), but can also contain texts, colors, a combination of other expressions, fields, etc. The point of using variables for expression is reusability, and also improves performance.
Quick use case: you have in 4 different charts a sum of Sales by category, department, product and country. Instead of using sum(Sales) in each of them, you can put the expression in a variable and use $(vSales) for all charts. If the expression changes to sum(Sales)+sum(Returns), you will only have to change the variable, not the expression in your 4 different charts.
That's just one use case among hundreds.
Hope it helps,
Sorin.
