In order to preview your print in PhotoShop it is absolutely necessary to soft proof. Think of it this way - let's say we have 3 different printers, or a printer with 3 different types of paper like glossy, matte, and newsprint. Sending the same digital file to each of these combinations of printer and paper is going to result in different output. Simply opening a file in PhotoShop and viewing it does not show what the print will look like. Only if you are soft proofing the image does the individual printer become part of the equation.