The hope publishers have is that it slows distribution a bit and they use it as attempt to manipulate people to allow stronger punishment of infringers.
But the watermarks really only matter when they get illegally copied around, and don't hurt the owner in the way more restrictive forms of DRM do.
Watermarks are an attempt at reproducing rules of physical space in digital space.
And yes, it's still trivially copyable, but that makes it better in any way than DRM: it can be traced back to the original owner but doesn't make it unnecessarily hard to read it on multiple devices.
But hardly any uploader would keep watermarks in, which makes them more of a way for faithful users to not give the book I.e. to friends since there is a visible reminder about being your personal copy.
Also, I specifically would not abide to any kind of law saying I cannot give a book I bought to a friend.
That's why, from publisher's view, the second point is so important: Remind readers all the time that "it" (without going into details) is illegal so they continue buying.
That's right. They are trivially copyable, but the copies are easily traceable. Compute now?
Except they often are visual. "Owned by X" and such.
Because digital things can be copied for free without loss of fidelity.
> Your printed books aren't. Watermarks are ugly.
A digital watermark can be invisible.