It's really a bit confusing, I've been told this a lot, so I've put together a small guide to try to help put it into practice, but it's a guide that focuses more on the asset's mechanism.
If you want to delve deeper into how 'sprites behind background layer' works, I recommend the phinioxGlade's tutorial on Github. The tutorial goes into great depth on the usefulness of the function, as well as giving some very useful tips on its use.
Link: https://github.com/phinioxGlade/gbstudio-background-sprites-tutorial