Why do designers request psd files

I hope its okay to ask this around here, was not sure where such a question would fit.

I am not a Graphic Designer, but a mere programmer :) I do have basic knowledge of operating Photoshop (watched a VTC once). Recently I started working with local designers and I have a question about the product they provide. (the PSD files)

When designing graphic for a website / application with multiple pages/screens, the designer will work with layers for each screen, and layers for each object in that screen.

The question would be, is it normal to ask for the design in a single PSD file? or it is better to ask for it in multiple PSD (one for each page?) it makes sense to ask for it in multiple so you can keep the slicing information for each screen, or maybe you can store multiple sets of slices in the same PSD for different sets of layers?

What is the standard for receiving a design?

I am pretty much asking from a customer perspective how to get the most out of the design I am buying and what to ask for so I can make small adjustments myself if required.