Bathroom Sunken Space for pipings and other Plumbing Works. Though it is not very common these days, yet this practice helps us in easy repair works in case of blocks and also to add extra fittings in the future.
INTERIOR AMMM by architecture, via Behance - Narrow vanity alongside the tub. This is what we have only our sink is centered (way too close to the toilet). I was having trouble finding images of off center sinks in this configuration.