there are thousands of ways to use a variable. I'll give two examples to get you started.
Theme: when using, custom, colors it's quite a job to manually change all objects. Instead - define a variable to hold your color definition and you only need to change that to affect all objects:
SET vColor9 = 'RGB(221,238,252)'; //Light blue Then use with $ expansion in calculated color $(vColor9)
Lets asume you have a chart that should only show if certain conditions are met. A home made tab perhaps. Then you assign a varable the tab value and set a show condition for the chart.