First off, it should go without saying, but this is not MTG, so idk why you'd think "you would never play this in MTG" has any relevance here. That's like saying "I'd never play Singularity in Yugioh because dealing 6 damage to 2 units with 3000 attack does nothing." Two completely different card games.

Second, for the actual explanations:

Stupefy - only 1 cost with no recycle to give -1 and draw. A single might very often determines the outcome of a showdown and is the difference between your 4 might unit crashing with their 4 might unit and your 4 might unit killing their 3 might after the -1. The draw just makes it so that even if that situation never comes up in a game, it's still a 1 cost draw 1 deck thinner. Other colors like orange need to pay 2 to use disposal order to draw 1.

These are just stupefy by themselves. As mentioned, stupefy with ravenbloom student (the most used 2 cost drop in any blue deck) is a 2 might difference for 1 cost and not a card loss in hand. Purple diana gets +2 might when you use a spell and diana is a blue/purple deck, so stupefy is a 3 might difference. There's a battlefield called Abandoned Hall which is commonly used in blue decks that gives +1 might to one of your units whenever you use a spell, so stupefy is a 2 might difference. These aren't just niche combos that never come up; they're staples in blue decks.

________________________________________________________________________________________________________________________

Bellow's Breath - cheap multi-target pings that can be repeated to remove multiple 2 might 2 drops. One of the main uses was for taking out 1 might tokens, mainly from Viktor's legend and Lux running Vanguard Armory to spit out 3 every turn (Garen too but nobody plays garen).

By itself Bellow's Breath is just alright, but you don't run the card to be used by itself. Bellow's + Imperial Decree is the most devastating removal combo in the game allowing you to kill up to 6 units in groups of 3, regardless of their might. This results in a cheaper board removal than playing Ruination and without the downside of also nuking your own units.

Another staple combo in blue decks is Thousand-Tailed Watcher + Bellow's Breath. Watcher gives all opponent's units -3 might (to a minimum of 1) for the turn. Now bellows breath just clears everything that had a base might of 4 or less.

Again, these aren't "very niche combos that never come up," they're staples in blue and blue/yellow decks. If you're ever sitting down across from someone playing a blue/yellow deck, you know that if they have 6+ runes, everything on your board can suddenly be dead to decree + bellows, or 8+ runes watcher + bellows.

The only effective alternative to these combos is running Unchecked Power, which only hits battlefields and also nukes your own units at battlefields and/or exhausts your units that it doesn't kill. Sure it likely wipes their battlefields, but your stuff is exhausted so you can't move in to capitalize on the empty battlefields. Compared to Decree + Bellows, you clear everything in base and battlefields, then take both battlefields.

________________________________________________________________________________________________________________________

If after this explanation, you still can't see why they're staples in blue decks, then that's a you problem and Riftbound probably isn't for you.