Yes, rendering and visualisation in general.
They should be visible in any (most?) viewport modes that would support them. Shaded, Rendered, anything that can handle meshes.
Either the meshes are calculated during viewport redraws (not an option for most of those since it would slow down the refresh rate significantly) or they are calculated once and cached as meshes. They are not stored as separate objects with layers and names and stuff, just as pure geometry.
The purpose of these tools is not so much conservation of memory, as it is conservation of intent. The underlying geometry does not need to be changed in order to make it ‘look’ like you want it to look. Curves and surfaces can retain their core shape, while appearing to be more complicated.