It's confirmation that the vaccines work the way we expect them to work. Also, this may help identify people that need a (or more) booster shots, or people that should be more careful even though they were vaccinated. Also it provides an explanation for people getting Covid after vaccination (breakthrough Covid as they call it here). Now you can say to people that scream: "Look they got vaccinated and still got Covid!" what likely happened and why that doesn't mean the vaccine is not doing what is should, in the population.

For example the people without (or lower concentrations of) antibodies may share certain traits/attributes (like recently undergoing chemotherapy I read in this thread somewhere), if we know that, we can avoid vaccinating these people too early (and thus needlessly) after treatment and we know we should protect them in another way.

It may all seem obvious, but I can tell you (as somewhat of a scientist myself), that any of these things could easily have been found to be not true and we'd have had to deal with that. Biology is complex, if we would have had to change models of how we think the vaccine works, that could have real world implication for how we deal with (partially) vaccinated populations for example.

I wonder if one's reaction to the vaccine is a measure for the magnitude of one's immune reaction and related to the effectiveness of the vaccine. I for one slept for 14 hrs after my first shot and spend another half day in bed feeling like I had the flu after the second. I'd like to thing I'm well protected now, but who knows. Only studies like this one can prove such hypotheses.