Dynamic variables are SnapTest's way to add variables "on-the-fly" while a test is running. They exist because sometimes it's required to pull in some data that hasn't been generated before test runs.
While testing a store checkout, sometimes the tax or total's can be unpredictable. Store the tax amount from a UI element during a quote/preview screen, and assert that the confirmation page has the same tax amount displayed.
Dates and times that are randomly or unpredictably assigned can be confirmed by storing the text into a dynamic variable.