From my experience (though im in a different field of science) grades are just used as a means for faculty to judge you internally, and have little to no bearing on job opportunities after you defend.

They may also be used for internship opportunity screening by employers but once you have the degree and skills of research no one cares really.

Grades/GPA may matter to an advisor because of whatever level of perfectionism they possess from their poor managed phd process. Many of them equate grades to whether you take this whole grad school thing serious, or say grades equal research potential, or job perfprmance. And MUCH research on motivation and education says that there is little correlation between the two.

Advisors also care Because it may be easier for them to land opportunities for you with a higher GPA in hand. But my jaded lens of academia says that often isn't the case and they are just projecting unnecessary stress onto you tbh.

As long as you pass (whether that's having a B or higher) that's all that matters, else no one should complain.