The planes are the shared backdrop of the D&D multiverse. It's where a lot of important monsters (celestials, fiends, Elementals and fey) come from. Three campaigns require them (Planescape, Spelljammer and Ravenloft). A lot of important multi-campaign baddies (like Orcus, Lolth and Tiamat) live there. And many of the D&D APs involve the planes either directly (Descent into Avenus, Wild Beyond the Witchlight) or indirectly (Princes of the Apocalypse, Tales from the Infinite Staircase).
And the page count is about on par with the 2014 DMG to boot.
I know your status on teh loer when it comes to core books, but the planes are a huge part of the shared D&D lore. As adventuring sites or as home for many threats to the players and their home wolds. And like the lore glossary, it allows WotC to say "the cultists are preparing a ritual to open a gate to Avenus and summon Tiamat" without having to explain yet again where Avenus is and who the F is Tiamat.