A structure is parenthetical if you can take it out of the sentence, and this sentence's intelligibility will not be affected in any way (that is, the sentence still makes complete sense and remains grammatical).
In this case, the sentence doesn't work without the "or", but it could work without the phrase "what is different". Although I do think the sentence is a bit awkwardly worded (this happens even to native speakers, and that is why all publishing houses have editors, but even editors are not infallible).
What the authors mean is that all humans
seek a worthy goal
meaning that all humans do it whether they want it or not; it is an ingrained trait,
or that they
ought to (should) seek a worthy goal
meaning not all humans do it, but the authors think they should; that there is a moral imperative that should be imposed on hmans. And they want to emphasize that these two states of things are different, that is why they inserted this caveat into the sentence.