[vbezhenar]

It's about heat. Cable has some resistance and emits heat. This heat has to dissipate somewhere. If cable is rolled out, it'll dissipate heat to the air. If "power wheel" is not rolled out, cable will heat cables around. Outer cables will dissipate heat to the air, but inner cables will not. So with enough current and enough time, this thing will melt.

You probably won't have issues charging iPhone from this thing or powering something for few seconds, so no need to go crazy about it, just something to keep in mind.