Strictly speaking, they let you cast the spell for an alternate cost of zero mana
Echoing Decay is a simple little common that is functional as a removal spell (although occasionally awesome against your opponent's best draws), but has one real purpose - exterminate all of your opponent's small creature tokens
As a bystander to the format, I can say that before Modern existed, the Extended formatwhich encompassed anywhere between the previous 4-7 years of Magicwas much maligned
It reeks versatility
It doesn't target, getting around opponents that have hexproof, and if you have four extra mana, it'll mill each opponent for half their library, rounded up
However, for this particular article, I want to cover an Artifact Superfriends deck, and I don't think our deck gains much by going 5C