While Archeops isn't particularly powerful, and you probably will never have to attack with Archeops, the ability lets you take a stronger card and immediately attack with it
This plays right into the expend mechanic from the main set, which encourages you to spend at least 4 mana each turn
Tapping Aryel to activate either of its abilities while it's attacking doesn't remove it from combat
Admittedly, while both of these abilities are incredibly useful, Gravemother is often better suited to being in the 99
Leyline of the Voids faults here are obvious, and its effect is such that casting it normally is rarely going to do anything of note, but Rest in Peace is a bit more counterintuitive
That said, if you need marked cards for any of your work, that you don't want to baby and keep in pristine condition, THESE are the cards to get by the brick