I use OmniGraffle for two things - UI mockups that includes click events. These are used to show clients website concepts in both mobile and desktop format. The second is to whip together network diagrams and explanatory charts for clients. I’ve used Viso, XYZ Microsoft product flavor of the day, and various free online versions, and it’s almost shocking how there is no comparison. Omnigraffle rewards anyone by allowing complete novices to hop in and make an outstanding flowchart; but the minute you start peeling back the layers of tools you realize you’ll spend a lifetime leveraging this application in more and more powerful ways. Worth many times what they charge for it. If you want free, go suffer with some other products. But if you are a professional who needs a real tool, then look no further.