Volume, and darkness.

Neither available above ground.

The garden keeps its space.

And the water keeps its condition.

Both passively, with no treatment.

Because it decides if the supply is real.

Several thousand litres bridges a dry spell.

A few hundred does not.

Above ground that size is unacceptable.

Buried it is a cover on a lawn.

Algae need light and get none.

And underground stays cool all year.

Which slows everything biological.

Stored rainwater keeps far better.

Than in any surface tank.

Because an empty tank floats.

In wet ground it lifts out.

Of its own excavation.

Which happens when it is emptied.

For cleaning, not at installation.

Which tank is suitable at all.

A high water table or heavy clay.

Or a position under a driveway.

Each needs a different specification.

Survey before ordering the tank.

Not unless it is rated for it.

Vehicle loading is a stated rating.

And a garden tank does not carry it.

The distinction is routinely ignored.

And the tank deforms under load.

By pump, always.

Submersible in the tank, or suction above.

Which makes the pump part of the system.

Not an accessory to it.

Provide access to maintain it.