Watermarks are typically used by people for a couple reasons:
First, a professional/commercial photog can have good reason to "protect" images from being copied and used, either because of the nature of the content or simply because the photos are the source of the photog's income and can't be just given away. If the photog has an interest in "sharing" the photos either on a web photo site or on a forum such as POTN, then using a watermark can hinder a person from grabbing and using that photo in some way that could infringe on the photog's needed use of the photo.
Other photogs may use watermarks even if they are not using the photos in a professional/commercial way for similar reasons -- they "own" the photos, the photos represent "intellectual/artistic property", and for others to "assume ownership" of the photos is at least unethical and, if used for profit, can actually violate copyright laws. Potentially, even an amateur can hope to sell a photo here and there, but it that photo becomes "public property" than the potential of that can decrease, so the photog might use a watermark to deter that possible misuse.
All that being said, there are counter-remarks about the usefulness of watermarks. For one thing, the photog will, presumably, retain the full-resolution photo files and only post images that have been resized for Web viewing which would not be good for prints of any size larger than, say, a 4x6 or a 5x7 without being seriously degraded in print quality, and the idea of someone getting "serious use" of an imaged resized to such a small resolution is, well, a consideration.
And, when you are sharing photos in a place such as POTN, the value of that sharing can be diminished by an obtrusive watermark to the degree that your "audience", other POTN members, may just ignore the photo and move on.
But, some people look for a compromise by placing a watermark that shows a very "light touch" (typically a very low opacity) and often in a place on the image that doesn't diminish the overall visible imact of the image.
Then, the problem remains that a watermark can be "photoshopped" out of the image, and that is the difficulty that people just have to deal with -- if someone has the skills to edit out a watermark, and has the motivation and is willing to spend the time to do this to an image that has at most say a 1MegaPixel resolution, well, there's just not much one can do...
As to POTN "rules", there is no rule -- you can use watermarks or not, it's personal preference. Sometimes if you use an "obtrusive" watermark, people may comment, but that's opinion, not "policy"...
But there is one instance where a watermark can run into difficulties, and that's if you enter some kind of contest/competition. Some contests specify "no watermarks" as a condition, which is their right, and for other more casual types of competitions, it may just be stated that watermarks may have a detrimental affect on the judging -- again, it's their right, no matter how much someone might complain.
I personally don't use watermarks. I don't shoot professionally. I have been rarely paid for photographing certain things but it never is the type of thing that watermarks would have been useful for. And, for shoots that shouldn't be "public", well, I keep them in password-protected folders on my Web host and so they aren't public! If I occasionally "share" a photo in a place like POTN, well, I am risking some kind of something, but then I still only am sharing a very low-res image, not a high quality multi-megapixel image.
But that's just me...