This is primarily to make accessing the deck's powerful Level 8 Synchro monsters easier when compared to a list that only uses Harmonizing Magician as a Tuner
If cards are part of a complete set (master sets, full holographic runs, themed binders, etc.), bringing the whole set in usually earns more than parting it out
While Ellivere equips creatures with a singular Role and generates value from attacking, Gylwain is less aggressive but provides more flexibility through a wider variety of Roles
Attaching it to Commander could reintroduce the random, kitchen table chaos back into a format that has arguably begun to calcify under the weight of so much official attention and continuous product releases
For the same four-mana cost, you can roll the dice to come up with your 7% chance of starting the game at instant speed, at no cost at all