Ah, this is a question that gets posed to professional photographers a lot, and something that they explain to their clients over and over again. So, I have decided to explain the importance of photographer's watermarks, when displaying professional photographs online.
While it may just be a snap of the shutter to you, when a photographer takes your photograph they consider it their ART! They take time to contemplate and work the lighting, the posing, the mood and feel of a photograph before even snapping that shutter. While it may not seem like it during a photo shoot, there is a lot going on behind the scenes, even if only in the photographer's mind, for creating that perfect picture for their client. Nobody would consider erasing the signature from a painting or drawing, however people think nothing of cropping a photograph to exclude the watermark the photographer has put on it, to brand the artwork as their creation.