I use to work for a large Dental Plan marketing website a few years ago and they had a large customer-driven website that sold Dental Plans to consumers. Their website started tracking users as soon as they hit their web servers, and then they logged everything they could about the user.
There are a lot of benefits for using session tracking for both the user and the website. Users can benefit from session tracking due to the fact that a website can retain pertaining information for the user so that they do not have to re-enter the same information repeatedly. In addition, websites can hold specific items in a cart for each user so that they can pay for all of their items at once when they are ready to complete their purchases. Websites can also benefit from session tracking because they can determine where a specific user came from and which advertising partner gave them a sale. This information is very useful when deciding on where to spend an advertising budget.
There is only one real disadvantage when it comes to session tracking, Users can not really control what is actually tracked by a website. Yes, they can disable cookies and this will help, but that means that no tracking can be done at all. Most sites require users to have cookies enabled in order for users to make purchases or login to their accounts.