Jason Snell, writing about the space-saving features in macOS Sierra:
The result is a whole raft of features in macOS Sierra that are all about freeing up more space on your drive. Some of the features work well, while others seem buggy or poorly implemented. macOS Sierra also changes the rules about how free space has been calculated on the Mac since the very beginning.
If you’re running Sierra, you should read this before checking any boxes related to these features.