Hi Jorge,
PSP is nothing but a bunch of software/Module updates what HP provides for Windows and Linux OS. As per HP to support with the HP hardware, the OS should have PSP.
In PSP certain driver modules are also included which will replace Redhat provided drivers. for eg NIC modules. Incase if you have some trouble with your NIC driver and you want to get the original one back the only way you have is a kernel upgrade.
You can go through the PSP doc, In the PSP 7.8 I think the NIC modules etc are excluded.
Thanks
Adithyan.
Keen to learn HP UX