The first seven of this type of pyramid gave a permanent Egyptwide boost to vegetable harvests (+1 per pyramid, up to +7 total). Seven Pyramids of the Fertile Land have already been built, so Egypt has already received the maximum bonus from this type of pyramid.
In Tale 1, it eventually became possible to rig a Pyramid of the Fertile Land with a larger version of the Set's Ladder puzzle. This became the basis of the sixth Thought test.