Variables are a powerful way to customize your dashboards' filters (and much more).
You can use it for advanced filtering. For example, you can use it to create a table displaying sales of a given year N and the year before N-1, with a filter to let the user choose year N.
You can also use it to create related dashboards. For example, a global dashboard can have a table showing all the sales per business unit, and links in each row to open a dashboard focused on a single business unit.
If you need to create not-so-simple dashboards, it's a must-read.