Lock and key is a widespread, but somewhat controversial design practice. The basic implementation includes a literal use of a lock and key. You restrict access to a certain area (or reward) until players find the key. To do so, they need to venture elsewhere and complete a set of tasks, advancing the storyline or their character.
Turning away your players and asking them to come back later is a form of so-called backtracking. There is value to it but you have to be careful as extensive backtracking and having to manage a list of uncompleted areas can be incredibly frustrating.
One obvious use is to prolong your content's lifespan. You may have spent a long time crafting a beautiful set-piece or a highly functional area, and ...
