I am a big fan of Ruscha's artwork, and it seems his photography is not very well known as it is overshadowed by his paintings. But that book and other works like "Every Building on the Sunset Strip" were influential to people like Shore and to non-movements like the New Topographics. I love Shore's work too, as well as George Tice's Urban Landscapes.
For me, there doesn't have to be any kind of "purpose" or underlying message. These photographs can simply be about a place in time. I never get tired of looking at them.