In gameplay terms, SP cards dont provide new mechanics (they have the same game text as the normal version of the card), so their importance is equivalent to their regular version for playing
If youre just trying to turn that Avacyn into a couple of Thought-Knot Seers, this may be your best bet
If I'm ramping, I want the most mana for my buck, so now I'm looking at Worn Powerstones and possibly even Hedron Archives, skipping the possibility of getting my Commander early in order to get more mana for bigger stuff
They feature a texture, unlike Illustration Rares
Whether you want to respond to casting the spell, the enters/leaves the battlefield effect, or the triggered ability to sacrifice it
With the latest addition to one of the best card games leaning hard into a retro horror aesthetic, theres plenty of references to spooky cinema across the MTG Duskmourn set