737 Classic Cabin Pressurisation

[Alltheway]

Dear All,

I'm completing differences training from the NG and trying to gain some understanding for the following procedure

"If the Differential between the selected LAND ALT and FLT ALT is less than or equal to 7.8 psid (Or 7.45psid for FLT ALTitudes less than 28,000ft), the controller programs the Cabin Altitude at cruise to slightly below the selected LAND ALT." Boeing CBT

Any expansion on why this is performed is greatly appreciated.

Thanks.