Caffeine is the main ingredient of all stimulant based preworkouts. It is the main reason you get all of that energy and focus when taking a PWO, but it is much cheaper to just buy ingredients like caffeine individually. You can also get the dosages right, instead of paying for expensive PWO supps with underdosed or mixed dosed ingredients.
Supplement in tablet form like this if you don't like the taste of coffee, or if coffee makes you.. need to do other things than work out, or so that you can get a precise measurement (like 200mg).
I use it in conjunction with 5g of creatine (daily). Keep your liquids up!