Approaching it from a time standpoint, let's imagine that you have hired a photographer who has work that you love. This photographer is traveling up to an hour to your destination to photograph your session. Here is an example of a time break down:
Booking - client contact time and paperwork - 30 minutes
Pre-session Preparation - equipment and back up equipment checks + vehicle checks - 30 minutes
Travel - up to one hour each way - 2 hours
Session - meet and greet, actual time spent taking photos - 1 to 2 hours
File Management - uploading and backing up images - 30 minutes
Image Editing - selecting the final 30 or so images from 200-400 that were taken, cropping, color, minor blemish removal, white balance, contrast and sharpness editing - 4 - 16 hours
Preview Gallery/CD - constructing online preview gallery - 30 minutes
Ordering Session - preparing products and loading images into ordering program, and creating final order - 1.5 hours
Order Assembly - preparing digital files for print, filling out order forms and sending to suppliers, putting together albums and other products that require assembly - at least 2 hours
Miscellaneous - usually client contact time, emailing, phone calls, organizing and discussing orders, keeping client updated on order progress - 30 minutes
In this example, the time spent per client can extend past 20 hours. This is time dedicated only to one session. When the photographer charges a session fee you are not just paying for two hours of their time, you are paying for multiple hours of work to complete your session.