1) in certain scenarios, due to higher FOV on 16:9, you are able to see the enemy before you would be able to see them on 4:3 = more time to react
2) overall the game is faster on 4:3 - this includes your movement, the enemy's movement, etc. - while this MAY TECHNICALLY JUST BE A PLACEBO, it's a placebo that actually practically works (hence why I don't consider it as a "placebo", whereas "thick models" are truly just a placebo), take this as an example:
- you are playing old D2, you are T spawn with an AWP and you are holding the mid cross, the enemy crosses and you are about to take your shot, on 4:3 your enemy is WIDER but FASTER, meaning you will have a harder time reacting in time, whereas on 16:9 he is THINNER but SLOWER, meaning it will be easier for you to react in time
not so hard to understand, is it?