Because it is a wet wall carrying tile.

The board must resist moisture.

And be rigid over a framed cavity.

Standard board flexes under tile.

Which cracks grout within a year.

The tile weight, not the board minimum.

A tiled prewall is heavy and rigid.

Board fixed sparsely will move.

And movement shows in the grout.

Follow the tiling requirement.

It is the only access to the cistern.

The valve and mechanism come out through it.

Cut too small or in the wrong place.

And a part will not come out.

Which is found years later.

Yes, they supply a former for it.

Positioned by the frame itself.

So the opening lands where it should.

And is the size the plate expects.

That is much of what the kit is for.

Because frame members are not studs.

They are not at plasterboard centres.

So there is nothing to fix to in places.

Battens give a proper fixing grid.

Across the whole panel.

Board ends up unsupported.

Usually right over the cistern.

Which is exactly where people lean.

And where a plate is pushed daily.

It flexes, then the tiling cracks.

It becomes a horizontal wet surface.

Things get put on it and spilled.

So it needs a proper finish.

And a fall away from the wall.

Rather than a cut board edge.