Since you can run two of them, this helps to avoid them totally outmatching regular Commanders
When a single copy is coupled with pressure against creature decks, Submerge does an excellent Time Walk impression while also being able to do cute tricks against decks like Lands (dealing with Marit Lage tokens can be troublesome) and Knight of the Reliquary (Submerge the Knight in response to their activating her ability
Thicker cards tend to convey a sense of quality, professionalism, and attention to detail
Its not something you should expect, but its a nice bonus if lucks on your side
When building around it, look for lifegain, artifact untap effects, blink effects, and payoffs that convert extra cards into meaningful advantage
Why would you want to pay the plot cost and cast the spell later instead of just casting the spell now if the costs are the same