​​​​​​Why is there a groove line on the "F" and "J" keys on keyboards?

 ​​​​​​Why is there a groove line on the "F" and "J" keys on keyboards?

The bumps or nipples found on the F and J keys on the keyboard helps users correctly position their left and right hand on the keyboard without having to look at the keyboard. For example, without looking at your keyboard you should be able to feel the keyboard and correctly position your left index finger on the F and the right index finger on the J. From this position, you should be able to locate the remainder of the keys on the keyboard.

A phone having a hardware numeric keypad has something similar. The digit 5 also has a bump. It is also used for the same purpose. One can easily and quickly enter the digits without having a look at the keypad.