I wonder why Mark cards weren't up on these subforums until now...
Anyways, Mark cards function as a normal pillar, except that the unupgraded version can stack with the player's Mark if the two are of the same element. In that case, the Mark played becomes indestructible.
So, its only special use is to counter EQ and other permanent controls, but people go crazy with the Marks due to their rarity. They're even rarer than Nymphs, since Marks can only be won from official tournaments and leagues.
As far as I know, the only publically known person owning a Mark of Gravity is Rastafla, along with various other Marks (which is the reason why it fits for him to put up the Mark cards topics). I myself own one, and am planning to receive two more from my previous League win code (that I still haven't received for like 4 weeks).