The space arithmetic
A twin mattress claims about 27 square feet with walking room. A loft returns most of that footprint to the room. Here is the arithmetic, done with this catalogue’s own dimensions.
The average loft footprint in this range works out to about 28 square feet. Put a desk under it and the desk’s footprint drops to zero; put storage under it and a dresser’s worth of floor comes back. In a 10-by-10 bedroom that is the difference between a bed plus a desk fitting, and one of them living in another room.
Corner frames play the same trick sideways
The corner designs wrap shelving around two sides of the mattress. The shelving occupies wall the bed was already touching, so the bookcase costs no new floor at all. That is why the corner line reviews the way it does in small rooms.
What the arithmetic does not say
Height. A loft solves the floor and spends the ceiling, and whether your ceiling can afford it is a separate check: run your room through Ceiling Fit before the footprint argument convinces you.