Firstly, its a 4-mana spell for blue/red, and it has flashback for a second bite at the apple
A lot of (unfortunately Red) cards put elemental tokens into play repeatedly and can do it multiple times in a turn, digging you deep into your deck and drawing a ton of cards
Secret Lair x Hatsune Miku: Digital Sensation featured six cards including a copy of Chord of Calling as well as a copy of Sol Ring
Accumulate Wisdom allows you to draw three cards for just 2 mana, if you have three or more lessons in your graveyard, so its an interesting payoff for a spellslinger deck filled with lessons
7 Triplicate Titan While Triplicate Titan doesnt have an enter-the-battlefield effect, it remains an incredible creature to bring back at any point in the game thanks to a great death trigger
This may have something to do with less people "gathering" to play, but even so, some players are left pining for many cards they couldn't use over Arena
