When creating my original application viz's I was not aware of parameters or what can be achieved with them. So whenever I came across a dashboard that allowed you to select an option or press a button and completely transform the data I was left bemused and I assumed the creator must have done some Tableau wizardry to create it.
In reality however, using parameters is not as daunting as it seems, and once understood, you can really transform your dashboard and by adding lots of interactivity that will improve the user experience of your end product.
Creating Parameters
When creating and implementing any sort of parameter you will follow the same basic steps, firstly you will need to create the parameter, here you can name it as well as configuring the type of parameter you want.
Once you have your parameter you will want to have it visible by your data by right clicking it and selecting 'Show Parameter', this is what will allow you to interact with your viz so that it can affect the data being shown.
Next you will need to create a calculated field that tells your parameter how to interact with your data, common logic statements are 'CASE' or 'IF' but it always depends on what you want your parameter to do.
Finally you will need to put that calculated field onto your data, where it goes depends on how you want it to affect your data but for example you may want the parameter to affect the colours on your viz or perhaps you want to put it directly onto the rows or columns.
Adding Interactivity
When you have parameters set up correctly you can have them appear on your dashboard and use them to interact with your data. This can be used in many different ways, create buttons that completely transform your data, use them in combination with containers to allow you to filter between visualizations, or maybe use them to change the way data is ordered or labelled in a chart or graph. There are countless ways you can use use parameters to add interactivity to your dashboard and the best way to discover them is to look on tableau public and find inspiration.