Trickery, misdirection and riddles are tools for any conjurer worth their salt. History is riddled with examples of cunning characters using cryptic means to hide the real meaning of what's going on.
For a long time, this was the cause of a lot of rules headaches. Opponents didn't even have to show the face-down cards after the game, so an unscrupulous player could technically cast a basic land under the Mask pretending it was a creature to help empty the hand in face of a Black Vise. Really against the rules of course, but unless a judge was close at hand you had no way of knowing.
That problem was addressed with the release of morph creatures, and the Onslaught rules specified that you had to reveal any face-down creatures with the morph ability at the end of the game. Humility from Tempest made this a little more complicated however. Every creature ability was removed with Humility - the face-down creatures technically didn't have morph when it was in play - so would you have to reveal them at the end of the game anyway?
Over the first three years of its existence, the Illusionary Mask didn't see much play at the top tables. New cards are constantly added to stir up the meta of course, and in 1996 Illusionary Mask met its first BFF.
Detractors could argue that this removed some of the mystery from the mask, as you pretty much always knew what 1-drop creature was hidden under it, but it was still a cool synergy. Look at this sweet Vintage MaskNought deck (as shown in Stephen Menendian's History of Vintage):
Of all the cards in ABU, Illusionary Mask may be the one with oracle text furthest from the printed text, just to make it work with current rules. These days, creatures hidden under the Mask are always cast as 2/2s without any abilities, just like any other face-down creature, which leads to a few strange interactions one might argue goes against "the spirit of the card". Other than removing all the strange static abilities, the creatures now have power and toughness that can be interacted with.
Why does that matter? Well, for instance a card like Infest can now kill all the face-down creatures, which it couldn't before unless they had a toughness of two or less when face up. Whatever stats the creature may have when we see it face-up are ignored as long as it is hidden. That removes a lot of the mystery around a face-down creature as we now know what they are; 2/2 colorless creatures with no abilities nor mana cost. Hellfire will destroy a masked Juzam Djinn for instance, which is disheartening for so many reasons. Explain to me how Demonic Hordes can be killed by Hellfire or Terror
but survives both a Cleanse and an Exorcist just because it was lurking
behind a mystical mask.
Onslaught opened the door for face-down creatures with morph triggers, instantly upping the ante for the tricksters among us. New sets will keep coming and eventually we might see a card that just begs to be played face down for its mana cost and then flip up for free. Dreadnought might have been the first tournament-worthy partner, but I am certain the Mask will find new friends as new cards keep entering the scene.
One last thing before we go, as it would feel amiss to end a 93/94 post with a picture of Hooded Hydra rather than some sweet 93/94 tech. This seems like a better note to wrap up on: