Creating a folder is a good way to separate and organize your web content and should be used to provide an ease of navigation. An example of when using a folder is a good idea is in the case that you have many files that are specified by date (i.e. 2010-file1, 2010-file-2, 2011-file1, etc.). In this instance, it would make sense to create a "2010" folder and a "2011" folder for the files to be categorized accordingly. It's also a good way to separate your document and image files from the rest of your pages and keep things organized.

It's not necessary to create a separate folder when there is only 1 or 2 files that will populate it. For example, if you're only going to have 2 images on a site, it isn't necessary to create a separate images directory. However, if you plan on using a larger amount of images, creating a separate folder would be a good choice.