One of the things that a Game Master tends to find themselves with after a number of years is old or outdated material. Whether this is because you no longer play that system, it's been updated, or you've already played it with your current group of players is beside the point. The key here is that you're likely to end up with a number of books (or PDFs) that you've read once or twice, or even played, and which will then become so much dead weight.
And that is where the idea of recycling and repurposing content comes in. Look at your current campaign and game, and then see what you can do to take bits and pieces from elsewhere that you can use to create something new.
And to illustrate what I mean, I'm going to use a Pathfinder Adventure Path, one that's much-maligned with some of the books receiving poor reviews, and the Adventure Path overall just feeling a bit out of place: Giantslayer from Pathfinder 1
The series has some cool moments in the various books, but to show what I mean I'm going to go with the first one (Battle for Bloodmarch Hill) in the series — ironically the one that doesn't involve the "true giants" of the system.
In particular, the siege is one that can be used. Replace some of the monsters that appear and which are threatening Trunau - change the orcs to humans or elves, and the cave giant for a wood giant, and all of a sudden you have a group of (supposedly) good guys attacking. Maybe the heroes are even on the other side this time, and part of the group that's trying to attack the town of Not-Trunau, which is now inhabited by orcs or goblins instead of "good guys". Maybe the heroes are even the bad guys this time, and it's dark elves and their enslaved ettin that's attacking the city instead.
In this manner, you can slip already existing content into your campaign and reuse it without having to completely come up with it from scratch. And when reskinned in this manner, your players are unlike to notice that anything has been reused, allowing you to spend more time on other projects, or planning longer-term goals for the heroes to accomplish and challenges to overcome.
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