[mooreken]

For every pound of pressure the boiling point is raised 3 degrees. So if your car runs hot a 25 pound cap can keep the water from boiling up to near 250 degrees.
As long as the water does not boil there are no hot spots in the motor to cause trouble. Granted I don't recommend that high a temp under normal conditions, but it does give some motor safety if it does run hot.

Just don't try to remove the cap while hot....very evil things happen.