Buffers supply against interruption; elevation can also enable gravity-fed distribution. Note the asymmetry: distributing from an elevated tank is passive, but filling one is not unless the source is higher still, a roof catchment or uphill spring feeds it by gravity, whereas a well or a ground-level tank needs a pump to lift the water up.
Note: Cost, carbon and footprint are rated against the other options for this same need, not in absolute terms.
Structural support for a full tank at height (water is roughly 1 tonne per m3), and either a source above tank level or a means of lifting water into it
No sizing or quantities: how much a technique yields, and how much a household needs, both depend on the build and the people, so the catalogue compares options rather than dimensioning them. It carries no bill of materials and no build sequence, and it is written for an oceanic, clay-soil climate (Normandy, France). Regulatory notes are indicative and need confirming locally.