why does the letters F and J key have a line underneath on all computer keyboards? ?

The ridges found on the F and J buttons on a computer keyboard are designed to help users locate the correct keys without looking down.

The reason why only these keys have ridges is to help users place their hands in the optimum typing position.With your index fingers on the two ridged keys - your left hand covers A,S, D and F while the right covers J,K,L and colon.Both thumbs then rest on the space bar.Others adaptations have also been attempted to increase the speed and accuracy of a keyboard user, such as raising the edges of the A,F,J and semi-colon keys.

So that you can type the keyboard without looking down. They help you identify the location of the keys in the keyboard.

Home keys for touch typing. That is where the index fingers from each hand rest.

This helps users to place their fingers on right spots on keyboard. For easy type writing this is very handy. 

The ridges found on the F and J buttons on a computer keyboard are designed to help users locate the correct keys without looking down. The reason why only these keys have ridges is to help users place their hands in the optimum typing position.

for visual impared people , the keyboard has a qwerty lay out , f and j have a line the can feel .its so they know where the other keys are , but your not blind are you .thats why windows made the screen in larger fonts  .atleast you have learn something .

Back in the old days of the typewriter, there is the so-called Home Keys where each of your fingers is assigned a letter (or symbol). Excluding the thumbs which both go on top of the spacebar, the rest are spread out consecutively over A, S, D, F, J, K, L, and ;. The letters F and J fall under the pointer fingers so those bumps on the modern keyboard make it easier for anyone who knows the system locate these two letters easily even when not looking at the keyboard (because that's how you're supposed to be doing it anyway).

When I was teaching Touch Typing in the lower grades, as a final test, I would wrap the keyboards with unused black garbage bags so my students would have to feel for F and J before they could start typing "The quick brown fox...".

So that you can find where to locate where to put your fingers for the home keys.

So when you're touch typing your (index) fingers can identify proper placement on the home row.

If you are a touch typists that is where you put your index fingers. With the line you can tell if they are in the right place without looking. Helps seeing impaired people as well.