Previews with a DataContext in WPF

Both the company I currently work for and the previous company I worked for use WPF extensively in some of their products. I recently ran across an incredibly helpful bit of functionality that aids in designing controls and forms on this blog. The TL;DR is that it lets you specify the data context that's supposed to be associated with a window / control, and lets you preview the control at design time using design data. This is useful because it lets you get an idea of what the control is going to look like with real data, and can be especially helpful when trying to size controls on a form, or seeing what lists are going to look like, and will be a real time saver because it means you no longer have to compile and start the application in order to view styles on things like DataTemplates.