The primary concept of free endless games is that they are designed in a way so as not to have an end. When a gamer starts playing any of these, the game usually offers a plentitude of goals to reach: development of a character, new skins, apparel, surroundings, weapon… But eventually, they are limited in their number (even if that number is pretty big). When any and all goals and offers are bought and used, any in-game offer is exhausted. It might be occasionally replenished with new updates of the game (which is most often true if a game is not an endless online game but some installable piece designed for a user’s device). Also, updates may bring changes to the gaming ambiance, giving it a particular holiday look (for instance, Halloween style). Eventually, however, a player makes progress much faster than any possible updates can replenish.
After all the achievements and procurable items are gained, the gamer is left only with the gaming process per se. In other words, they keep playing for the sake of playing. An in-game currency might still be accumulated, though, in many cases reaching completely insane numbers.
Some freely playable endless games are made in a way to make the process of reaching their goals much tougher to keep the interest of a player to them for as long as possible. Saying otherwise, something very cool might cost 1 trillion coins while every level brings you only a million (that would mean, you’d have to play a million levels to get there or use real money to make that purchase if you’re really allured by it).
Those online endless games that are really free to play and do not require money are usually imbued with advertising, watching which keeps it possible to deliver the game really free for all.
