What is the significance of monster subtypes?

Question

A number of subtype tags are added after some monsters' general classification. With the exception of Shapechanger and a few other specific exceptions these tags are linked to a particular monster type.

For example Fiends as classification sometimes have the subtypes Devil, Demon, and Yugoloth in parenthesis after their monster type in their stat blocks.

You can consider an imp, a quasit, a hellhound, and a mezzoloth. The imp is listed as Tiny fiend (devil). The quasit is listed as Tiny fiend (demon). The hellhound has no listed subtype and the mezzoloth has the Yugoloth subtype. There is also the unusual Gnoll Fang of Yeenoghu that is listed as Medium fiend (Gnoll).

This distinction is not afforded to every common language "subtype" of monster. Hags, for instance, have their own section in the Monster Manual but are only listed as Medium Fey or, in the case of Night Hags, Medium Fiend without any listed subtype.

The following tags are the subtypes I could find listed in monsters' stat blocks

General: Shapechanger, Titan
Aberration: None
Beast: None
Celestial: None, (Titan)
Construct: None
Dragon: None
Elemental: None
Fey: None
Fiend: Demon, Devil, Yugoloth, (Gnoll)
Giant: None
Humanoid: Goblinoid, Elf, Gith, Gnoll, Gnome, Grimlock, Human, Kobold, Kua-toa, Lizardfolk, Merfolk, Orc, Quaggoth, Sahuagin, Thri-kreen, Troglodyte, Yuan-ti, Aarakockra, Dwarf, etc...
Monstrosity: None, (Titan, Yuan-ti)
Ooze: None
Plant: None
Undead: None

With the exception of "shapechanger" that is affected by a number of spells what is the significance of these tags? Does a particular subtype having a tag (such as devils and most humanoids) have any particular meaning that is different from a variety of monster not having a tag (such as hags and beholders)?

Answer 1

These tags by themselves don't mean anything in mechanical terms

From the basic rules about monsters:

Tags
A monster might have one or more tags appended to its type, in parentheses. For example, an orc has the humanoid (orc) type. The parenthetical tags provide additional categorization for certain creatures. The tags have no rules of their own, but something in the game, such as a magic item, might refer to them. For instance, a spear that is especially effective at fighting demons would work against any monster that has the demon tag.

Given that they don't have rules by themselves, they are only included to work with other rules that refer to them, such as the various rules that specifically work with the "shapechanger" tag (such as the moonbeam spell), as you note.

Although most of these tags aren't used by any existing rules, they may have been added to future proof; for example, if one day there's a magic item that specifically does extra damage to orcs, then having creatures with the "orc" tag would be a good way of determining whether or not this magic item deals that extra damage or not. This is especially relevant for creatures like gnolls, where they might be humanoid or fiend, so this is a way for a game rule to go across creature types without making it effective against too many creatures (i.e. all humanoids and fiends).

As for why certain creatures have gotten tags and others have not, I cannot guess why the designers have made the decision to tag some monsters but not others. For what it's worth, I agree that tagging hags with a "hag" tag would make sense (especially since, as with gnolls, the tag would be useful to group together creatures of different types; fey and fiends in this case), same for beholders (even though in this case they are all just aberrations).

It might be worth noting that there weren't many hags or beholders in the Monster Manual, so it might simply have been that there weren't enough to justify it at the time, and by the time Volo's Guide to Monsters came out and introduced more, they'd already set the precedent not to tag those creatures.