Whenever I finish an artwork, I sign it and watermark it as a tradition. However, some people may not understand the underlying reason for doing so, since that takes extra time for what could be a useless endeavor. Truly, when you're posting art on the internet, some people may want to spend the least time possible between one post and another, and this may get in the way of the process.
Affirmation 1: Signatures and watermarks make the final picture uglier. Truly, they are placed over the finished art, which can be distracting and might take away the enjoyment of looking at the art from the viewer. As an art creator, you usually want to make sure that the viewer has the best experience possible, that is, to see the art exactly as you see it.
Affirmation 2: AI services can easily remove them. As AI computation improves, so do their abilities to remove signatures and watermarks without leaving many artifacts. This means the watermark is basically useless against theft, and thus, it's a waste of time to do so.
Affirmation 3: It's extra effort to implement a signature or watermark. Usually people want to post the art immediately as it is complete, to get the most reactions as possible. Sometimes, their way of working is already so streamlined that it's a hassle to remember adding signatures or watermarks.
Considering the points above, it makes you assume that adding such features on your art is useless, even detrimental, and that you should avoid doing it. However, I do it because this shows my care towards that I create, and that the art I create is my brand which cannot be fully copied, no matter how much the art thieves try to do so.
Counter to Affirmation 1: A watermark and signature is your stamp of quality—it means that you yourself created it, signaling to your friends and fans that you spent time and effort to make the art. This reason alone is enough for them to understand the existence of the watermark, and accept it. Very few people actually complain about watermarks being too obtrusive: as long as you integrate them well enough, people will understand it.
Counter to Affirmation 2: even if AI services can easily steal and replicate my art, this is still very effective against human thieves. Remember that human thieves still exist, and still do a sloppy job at stealing. If your signature is well-implemented, it can help filter ill-willed users out and help your fans recognize your work. Also, not all AI services are of equal quality: some cheapskates will use poor quality AI scrapers to gather your work, and your watermark might survive their edits as well. At the very least, it is also useful to signal to your human friends and fans that you do not support AI generation, which strengthens your community against this threat.
Counter to Affirmation 3: signatures can be designed and saved as assets and easily pasted over the work on a new layer, so changing your art routine to include them should be easy enough. More important than posting frequency, is the care that you have with your own work. People like downloading or pinning art pieces to admire later, and if your signature/handle is included in the watermark, it will allow them to trace it back to you easily, which in turn leads to more human fans discovering your work.
Thus, I strongly believe that even though stronger AI scrapers can remove them, watermarks are still useful to build stronger human connections and to loudly assert what you want others to do or not to do with your art. It is better to make yourself clear than not to.