Stores usually post large warnings about this problem, and that is why. Even so, mistakes can happen. For example, I had once used my wife's machine to purchase something from Amazon. On something like a Windows NT machine or a UNIX machine that uses accounts properly, this is not a problem. The accounts separate all of the users' cookies.
Accounts are much more relaxed in other operating systems, and it is a problem.
When you do that you lose all of your cookie files. Now when you visit a site again, that site will think you are a new user and assign you a new cookie. This tends to skew the site's record of new versus return visitors, and it also can make it hard for you to recover previously stored preferences. That is why many sites now store all user information in a central database and store only an ID value on the user's machine. If you erase your cookie file for How Stuff Works and then revisit the history URL in the previous section, you will find that How Stuff Works has no history for you.