Why were hatched polygons pours used instead of solid pours in the past?
An old-timer EE told me the following while I was learning PCB design...

In years gone by, when manufacturing processes were less advanced than today, the high temperatures involved in wave soldering a PCB would cause gas pockets trapped within the fibreglass weave between the conductor layers to expand.  If there was a solid copper plane above the gas pocket then it would bulge out and de-laminate from the PCB.

To rectify this, a cross-hatched plane was used in order to let any gas escape through the tiny holes instead of rupturing the plane.  Any damage could quickly be repaired by manually applying solder-mask to fill in the microscopic holes left by the out-gassing.

This is very likely a thing of the past as modern techniques are extremely effective and materials are of a higher quality.