The truth is everyone has cell phones and everyone has learned how to take adorable photos with them. But there are true perks to hiring professional photographers especially for engagement sessions.
The first reason is because this is a great chance to get to know the photographer you are considering for your wedding. It is a chance to see their style, how they work, how you all get along, and see their final products. As well as how they improvise for weather and other issues that arise.
Secondly it is a chance for you as a couple to get use to being upfront of a camera together. To learn to relax and pose together in flattering ways for the camera.
And thirdly it is a way to have fun as a couple and express yourself. To impart photo ideas and styles you love to your photographer and maybe even get a unique couple shots you never dreamed of.