Tucking something on ETB is incredibly strong, especially in the blink colors of blue and white, and doing it to each opponent is going to be at least passably good in EDH
It has no special rules meaning other than being a designation that the ability (and, theoretically, other effects) can see
It was a less aggressive but usable alternative to cards like Professors Research, which lets players draw seven cards but must discard their entire hand first
Call the Spirit Dragons | Illustration by Liiga Smilshkalne You win the game cards let you win in alternative ways besides milling, poisoning, or reducing your opponents life total to 0
Golem 4400/3400 25930 677 26233203 Mystical Beast Serket 2500/2000 26578 396 04087376 Ocubeam 1550/1650 31205 380 32886522 Blue-eyes Ultimate D
This is good card quality as well as a good discard outlet