Can't speak for the Whovian one, but the Reality Marble concept is a bit different than most "pocket plane" fantasy concepts as it's explicitly a projection of the creator's "inner world" (that is to say, the set of axioms, beliefs and associations most central to their mind/soul/self/whatevs) into a reality.
To use the most famous example, Unlimited Blade Works isn't a pocket dimension that creates infinite weapons (especially swords) on demand because Archer/Shirou wanted it that way, but because "sword" is what he is, his inner resonance, the expression of his soul. That's also why at various points in the game (bad ends, mostly), he would seem to be made of swords rather than just flesh and blood - the same reality marble was within him long before he could enforce it upon a space outside his body. On that note, Nrvnsqr Chaos deliberately keeps his own Reality Marble inside his body to use his own powers (since otherwise reality would deliver a big "Huh, WTF, this is impossible" to him).
(I am, of course, simplifying a lot in this explanation, because Nasu cosmology and magic rules are so insanely complicated.)
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To use the most famous example, Unlimited Blade Works isn't a pocket dimension that creates infinite weapons (especially swords) on demand because Archer/Shirou wanted it that way, but because "sword" is what he is, his inner resonance, the expression of his soul. That's also why at various points in the game (bad ends, mostly), he would seem to be made of swords rather than just flesh and blood - the same reality marble was within him long before he could enforce it upon a space outside his body. On that note, Nrvnsqr Chaos deliberately keeps his own Reality Marble inside his body to use his own powers (since otherwise reality would deliver a big "Huh, WTF, this is impossible" to him).
(I am, of course, simplifying a lot in this explanation, because Nasu cosmology and magic rules are so insanely complicated.)