Why do I need to have a mobile friendly / responsive web design now?

Google has once again updated its algorithm, stating that all websites should have a mobile friendly/responsive design (web pages adapt by responding to the device shown on). An example is when you view a website on a desktop, and then you can view it on a mobile device the same way because the window adjusts accordingly. These are now best practices in Google’s eyes, and many sites can be penalized if not willing to update.