First and foremost, it saves me time and mental energy by allowing me to leave the house without worrying about making sure I have all of the information I need. I often get in the car with just an idea of where I am going, and then find the address while driving and use the phone as a nav system. I use voice recognition so it generally only takes a few seconds and minimal distraction. Along the same lines, if I am out driving and I suddenly think of something I need to do, I can find the place I'm trying to go without having to return home. Finally, if I'm idle at any time I can use the phone to write and send e-mails (or other basic tasks) that I would have otherwise had to do at another time.
This is just one subset of time-savers, but I would say that it saves me about 15 minutes of time a day by itself.