Why would you tap your phone?
Did you know that your phone will try to set the focus on the object near the middle of your composition and possibly the object closest to you? But what if that’s not your intention. What if you want to create a rule of thirds photo and put the subject to one side in your shot?
What do you do? You tap your phone. Yes, tap the screen where your subject is and the phone will shift the focus and exposure directly to that area in the composition.
Want to recompose? Easy. Long tap on your subject and the phone will lock the focus and exposure. You can then recompose your shot with the focus remaining on your subject.