This type of bed refers to an extra bed that can slide out from underneath another bed. In the past, this was a common practice for parents to have a bed for their child. You can think of it as a pullout bed that can be hidden during the day.
Since the trundle bed is very space efficient, it is also flexible with the size of the bed. You could have a large main bed and a smaller, single pull-out bed. Both beds could also be the same size. This space-saving feature is very suitable for small Hong Kong apartments.
Additionally, if you ever need an extra bed for a relative staying the night, the need is temporary. The trundle bed is easily stowed away, so it's only taking up extra space when in use. If you want to have more kids in the future, the trundle bed may be a good starting point before they move to a bunk bed or other style bed.
No, it does not.
The bottom underneath the hidden bed can still have storage, like drawers. This setup does not detract from your overall storage capacity.
It's very good.
Holetto uses 18 mm thick solid-core plywood for our cabinetry. The load bearing capacity is really good. With these materials, there is not a risk for bending or deforming. Even in a humid environment like Hong Kong, it can easily last 10 years and up.