It's necessary for someone learning iOS, but does one think that someone with several years of iOS experience really needs to see that line to know how to return to the Home Screen or App Switcher? Control Center has a line only on the lock screen, but everyone knows how to access Control Center everywhere else. Notification Center doesn't even have a line, but we all know how to access it. We can also access the app switcher from the home screen despite there not being a line. Apple's use of lines to indicate slide in gestures is inconsistent.
But to the point, considering that iOS doesn't actually need a visible line to recognize slide in gestures, why is a visible line a necessity?