I posted it because I see so many image posts that are absolutely gigantic. Using this format make pics more manageable and legible.
Eventually attachments have to be deleted because there is only limited space on the server for attachments.
If you want to post an image, put the url between the img tags, and then use the width or height attributes in the opening tag to set the width, or height, of the image (normally the image will maintain its aspect ratio if you just set one of the attributes).
For example, suppose you have an image that is 1,000px in width and you want to post it so it has a width of only 450px. To do that you would post the following:
[img width=450]http://www.mypicurl.tld[/img]