The isolation valve/s are primarily there to 'separate the bleed air ducts into isolated segments' (B12) so that a leak on one side can be isolated and not fed from the other. The Pack Off QRH allows air to be taken from the other side in the event of a dual pack off condition using one of Mr Boeing's less intelligible statements : Consider an alternate bleed air source if dual PACK OFF condition is a result of engine bleed air loss not caused by a duct leak or engine start valve failure.
So, eg, a L pack failure followed by a R Eng failure would allow you to open the isolation valve/s to drive the R pack from the L Eng bleed (or the APU below 17000').
The protection given by keeping the ducts isolated is more important than having both packs working.
Hth