In fields where this is the norm, though there are others where it is not, the publications along the way are normally closely tied to the dissertation research. Or, they might represent the essence of that research.
In the extreme case, the dissertation is little (or nothing) more that "stapling" together of those publications. Sometimes a summarization is required, but not always.
The publications might be partial results, or "side" results. These papers need to be sufficiently "novel" to stand on their own, but one normally learns a lot of things while doing a dissertation and many of them can be worthy of independent publication.
In some fields work is done in large scientific labs with a lot of collaboration. A student working in such a lab might get quite a few joint publications along the way. This might cover some fields other than the ones described above.
My field was mathematics, where this practice is uncommon, but my dissertation included a number of results (lemmas, say) that supported the main result and the results as a whole supported a complete theory. Had it been the custom in math, I could easily have published a number of those partial results.
One judge of whether a result is worthy of publication is whether it gives insight to others working in the field. The researcher him (or her) self gains those insights along the way and they can be worthy of sharing through publication.
But it isn't (usually) that the publications are independent of the dissertation research. That can happen on occasion, but usually they are tightly interconnected.
Note that the purpose of doing a doctorate and, within it, a dissertation is to teach the student to do research at a professional level. Publication is the ultimate "proof" of that. It also lets the research community as a whole, via the publication blind review process, to take on the job of vetting the research prior to awarding a degree, rather than leaving it to a few faculty members.