The planes are critical for helping players rationalize and organize information about some of the aspects of Dungeons & Dragons that are both the most fun and most confusing. The 5e writers identify these problems, listing them as the justification behind the minimum number of planes required for most DND campaigns (DMG, p.43): Where do Fiends and Celestials come from? Where do the deities “reside”? Where do mortal souls go after one dies?
Introduction to Planescape