Spoiler: many games include gameplay features which aren’t purely implemented for entertainment’s sake. Some, including experience grinding, upgrades, achievements and number building can be used to seemingly give a game more longevity and keep you playing. They trick the reward part of our brain to continue playing and make us believe that we’re accomplishing something.
Then along comes games like Orteil‘s Cookie Clicker.
On the face of it, Cookie Clicker sounds as dull as a video game could possibly be: you click on a big cookie image to bake a cookie in an effort to… uh, make a lot cookies for no reason or gain whatsoever. Once your click ten times during your first playthrough, you’ll have enough to purchase a cursor upgrade that will automatically bake one cookie a second. Then you’ll have enough for a grandma, who will be able to automatically bake even more cookies at a quicker rate.
Eventually, you’ll build your way up to purchase further upgrades like mines and shipment and even purchase upgrades for your upgrades that will boost the amount of cookies you’ll automatically make each second. Occasionally, a ‘golden cookie’ will appear somewhere on the page which can be clicked to either temporarily multiply your cookie production count or give you a one-off payment of cookies.
Some way down the line, you’ll be automatically baking over a million cookies a second in an effort to buy as many of each upgrade as possible, gain as many in-game achievements as possible and purchase all of the available upgrades in the store. It’s a game that tricks our brain in to wanting to play for hours on end in an effort to achieve virtual nothingness. Whether it’s specifically ‘fun’ or not, it is addictive as hell.
Coin Arcade 
What’s Cookie Cutter?
Whoops! I have no idea why I did that, thanks for pointing it out.