The reason for this has to do with the way Subject threads are related in many newsreaders. However, if the Pudding Fight turns into a discussion of Sexy Food, it's a good idea to change the subject line to reflect that. A lot of people use them to say who they are and a little about themselves. As a general guideline, signature files should be kept to roughly 4 or fewer lines in length. Going too far above that number is likely to get you an e-mail reminder or two from Callahan's people, and nasty flames from others if you post to different newsgroups.
Large signatures are another thing which cost many news readers precious time and money.
Occasionally people have a very large file of some sort which they wish to share. It may be a long story, or a graphics or sound file. This applies to graphics and sound files, which are almost invariably huge in size. There are newsgroups especially designed for such posts, and they form the only exception to this rule. You can find them under alt.binaries .