Depth is created with reverb and low volume in comparison to other parts of the song. In Logic you can add a reverb effect easily and adjust the settings to your liking and then lower the volume of that track.
This effect is desirable when you want to create the appearance of distance or want to record it in a certain style. Distance gives a song variation and separates different instruments to others. It’s most commonly used on keys or guitar to add small additional parts that don’t layer over everything and make it sound blocky.
You probably wouldn’t want to create depth on a main melody or bass. Creating depth on the drums could work to some degree.
Limp Bizkit’s ‘My Way’ has depth created by reverb, delay and levels in their introduction.
The delay and reverb on the guitar throughout the verses, intro and outro spaces out the song and gives other instruments room to have their own place. The bass in particular has been highlighted by creating depth within the guitar. The vocals in the introduction also have some depth as they’re quieter than during the verses and have reverb and minimal delay on.