They need to have some baselines for advice. So they can say "in the average game PCs will gain this much magic and you can account for that by doing Y". So things stay balanced. Which also helps for organized play, where they can say that "this campaign is Average Magic, so at level 10 you need to make X adjustments and at level 20 you need to do Y" to their campaign staff.Me? I've got nothing to say on the matter as my entire view of this is: If you're too stupid, dense and/or unimaginative to be able to see how/when adding magic items in your own low/average/high magic campaign works for your table or not, then you shouldn't be in the proverbial "big chair."
There is no reason, whatsoever, that I -or any DM- need to be told 'PCs should receive 6-8 permanent magic items over 20 levels. That makes the 'default' D&D game work. But if you want a lower magic game, then just give 'em 3-5. And if you want higher magic, give 'em 9-12."
It's also very helpful for new DMs who might need a little advice on what is expected and what impact it has on the game.