why does mel ops procedure require to cross supply?

[gearpins]

B 737-800 36-05 Engine Bleed Air Shutoff Valves(PRSOV)

OPERATIONS (O)

For left engine bleed unusable:

A. At altitudes 17,000 feet and below, operate the leftpack using the APU bleed air, the right pack using the right engine bleed andthe isolation valve CLOSE.
B. At altitudes above 17,000 feet, operate with the APUbleed air OFF.
1) Limit altitude to FL 250.
2) Operate the leftpack using the right engine bleed, the right pack OFF and isolation valve OPEN.

For right engine bleed unusable:

A. Limit altitude to FL 250.
B. Operate the left pack using the left engine bleed, rightpack OFF, and isolation valve CLOSE.
NOTE: At altitudes 17,000 feet and below, increased airflow will occur when flaps are extended (takeoff and landing) and the APU isused instead of engine bleed to supply bleed air to
the operating pack. Refer to Boeing FCOM, NO ENGINE BLEEDTAKEOFF AND LANDING SUPPLEMENTARY PROCEDURE.
greetings to all.
trying to get my head around the highlighted part
obviously I am missing something.....
why cant we simply use right bleed to supply right pack and keep it simple?
appreciate all help thanks
reposting to generate renewed interest.....
My best guess so far is zip.....
and that's worrying me....calling for B 737 veterans / experts....