Sometimes, it's because it's just powerful and wins games
Not necessarily good cards, but great if you like people asking to see your cards and getting Cheeto dust on them as they read your ancient cardboard relics
Pair this with a Phyrexian Altar and another zombie in play, and you're off to the races
Blue: Day's Undoing is a Timetwister that doesn't cost you a kidney, but does require you to run good cards like Leyline of Anticipation and Vedalken Orrery
Most drastically, it dictates what colour cards you'll play: you can only use colours that are in your commander's casting cost, or the cost of its abilities
Combine those two angles, and you have one of the most iconic enchantments in Commanderan engine that pushes any counter-based deck from steady value into explosive, game-ending power