Some believe it gives them security from image theft, like you i find it annoying and almost offensive to read half a profile made up with the statement. I picked this up from a US lawyer site about the need to state ownership of pictures, it was a reply to someone asking can they just take other peoples pictures.
But the Photo Did Not have a Copyright Notice On It! Then, if you want to use the photo, that should alert you to do some extra work to find out who owns the image. US Copyright laws do not require the author to include a copyright notice. Yes, having one makes it easier to find out to whom you need to go for licensing. However, the lack of a copyright notice does not mean it is in the public domain or yours for the taking.
Unfortunately despite the law of copyright being in place in most countries, pictures are taken and used regardless. Any picture you produce automatically has copyright as you are the owner, any one who uses it without you agreeing is infringing this. The statement does not need to be made on each profile, no doubt it is made on the site somewhere anyway.
I just added a short warning on my profile. The reason I did was that I see blogs that say they have downloaded pics on the web and if anyone wants them removed to let them know. Will it keep me safe? No. Will it stop someone who is kind and considerate (which I believe still exist "out there")? Yes.
Not much to add to the previous comments, except that much of those notices are crap, as I have seen warnings on pics that the poster definitely has no copyright ownership to. Some people think such matters, others try to add credibility with them, a few may think it looks cool to have such disclaimer and so on, so on.