I'm not quite understanding this. How does getting handed a .psd template help me make the app? Is this simply one possible alternative to the stock UIKit skin

I'm not quite understanding this. How does getting handed a .psd template help me make the app? Is this simply one possible alternative to the stock UIKit skin that's been packaged up as an idea to reuse? (Like Bootstrap, if Bootstrap was just a Photoshop document instead of usable CSS/HTML?)