Personally I am not the kind of "collector" that would pay such sums of money for a sheet of paper, either it be a movie poster or the declaration of independence, hence It would be very hard for me to get my hear around that rationale.
The thing is, at least for me, if I am collecting MOVIE posters, the movie of the poster will always be part of the deal, but then again, tastes are purely subjective, I am getting to an age where I find difficult to "invest" (*cough* waste! *cough*) +2h of my life watching, let's say, a superhero movie, very few make the cut because they are all about full on action sequences to an extent I get rapidly fed up with. BUT, some of the posters for those movies I find quite nice, for example:
(https://townsquare.media/site/442/files/2015/12/x-men-apocalypse-poster-full.jpg)
I do not own this poster, I have not seen the movie and probably never will because reviews are bad enough that it is not even one of the better superhero movies, I won't waste my time.
Still the thematic evoked by the minimalistic presentation does catch my eye... It is not a poster I would refuse getting if it came my way at a price I found to be reasonable.
I also forgot the "type of poster" appeal, which seems to be a more recent trend, maybe only from the 90's, which are teaser/advance posters. Graphically-wise they are always more minimalistic, many times purely to induce a certain suggestion, they barely if at all mention names, maybe sometimes only the director in order to ramp up the hype to some hardcore fans he might have but all and all that ends up, at least to me, as being a more artsy way of looking at it.
An example from a recent movie:
(https://image-cdn.hypb.st/https%3A%2F%2Fhypebeast.com%2Fimage%2F2017%2F05%2Fblade-runner-2049-posters-01.jpg?q=90&w=1400&cbr=1&fit=max)
Many times I find teaser posters to best released for the movie and some of my favourite.