[scottj]

When the heads are torqued down, the stresses on the block caused by the headstuds or headbolts will cause the cylinders to be out of round. Deck plates or torque plates simulate those stresses during the machining operation. What you then have are cylinders that are out of round after machining, when the deck plates are removed, but are round after the heads are installed.

The benefit is better efficiency due to better ring seal. Not necessary for a street engine, but better.
Scott