Why is there a groove line on the "F" and "J" keys on Apple keyboards? If you look at Apple's keyboards nowadays, you'll see that on the "F" and "J" keys there is a little groove line under the letter. 

Why does this exist? What purpose does it serve?
Actually, this is a feature of all keyboards, and has been for as long as I can remember. Most keyboards use a line, but older Apple keyboards used a dot.

The goal of the ridge is to allow people who type without looking at the keyboard to easily find the right keys to put their fingers on. These keys are referred to as the home row.

Both Qwerty and Dvorak keyboards (image) have these notches where you place you index fingers on the home row, which shows that the notches correspond to a location and not a particular letter.

Bottom line: If you don't look at your keyboard, these are an important feature to let you put your hands where you need to. If you do look at your keyboard, these don't help you.