Here's what MOST watermarks are for; you post a picture, comic, or gag-comic on the internet, onto Tapastic or a social media account. Someone thinks your art or humor or story is way cool, and they save the picture, and they send it to a friend. That friend sends it to another friend. Someone posts it on tumblr, saying they have no clue who drew it, but hey, isn't it cool anyway? Someone pins it on pinterest. by then, it's basically lost to the internet, with tons of people later image-searching it to find out who originally made it so they can see more content by that creator, but they can't, because all they can find when searching are the re-posts.
On Pinterest you can search ANYTHING, add 'comic', or 'drawing' after your search, and finds hundreds of thousands of drawings from Tumblrs now impossible to track down.
SOME of those drawings, however, will have a small, handwritten watermark on the side of it with that artist's online handle or their Tumblr name or whatever, and it's only THOSE drawings I can trace back to the original poster.
Can any watermark be taken off with photoshop, hell, with windows paint and a mouse? Yeah. But most watermarks aren't trying to protect themselves from that, because theft like that isn't very common and is impossible to stop when someone does have their mind set to it.
On that note, I DO disagree with certain types of watermarks. Those people who will type their name and ctrl-c it all over the picture without even lowering the opacity of the writing are just because ridiculous.
And also it should be noted that what I just was talking about with it getting re-posted on tons of sites only really happens with stand-alone drawings or gag-a-day comics. If someone re-posted a page of a story comic that barely makes sense out-of-context, no matter how pretty the art is everyone will immediately search the comic to make sense of what they've just seen.
If you ever choose to watermark your own work, I recommend a small, hand-written watermark somewhere on the drawing where it doesn't detract from the viewing pleasure. I find writing it along the side of a character instead of over-top said character usually looks the best. Maybe lower the opacity of the words a tad, too, and choose a color that matches the drawing (like, a black water-mark on a pale blue drawing looks bad. Go for dark blue instead). Thanks for reading my long answer XO