It really depends on the curve. Weed-out classes are famous for having extremely aggressive curves. You can be perfectly smart, be intellectually curious, and have a great grip on the material, but due to the competitiveness wind up with grades too low for, say, medical school. You need those A's and A+'s in the early courses to pad your GPA so that you can take the hit from taking harder more intellectually-demanding courses later-on if you're in a demanding major.

A potentially brilliant scientist isn't necessarily the best at memorizing every single idiosyncratic, arcane detail that the professor uses to differentiate an A+ from a B.

I got some pretty darn good grades in undergrad, and whenever I explained to a research PI how the sausage was made they were pretty disgusted by it, because it involves zero intellectual curiosity. Just dissecting and memorizing massive amounts of minute details from the lecture that have zero relevance to the real world. I didn't get those grades because I was the smartest guy in the class (I wasn't by a long-shot), I got them because I was a machine that resided in the same cubicle on the second-floor of the science library 7 days/week. There are plenty of people with lower grades than I got who would be magnificent surgeons or scientists. But, that GPA is a huge gatekeeper to those professions.

For example, the infamous med school STEP 1 exam is going pass/fail next year, because students have stopped caring about learning about how to practice medicine in the real world, and have started grinding thousands of ANKI cards every day to memorize infinite details about weird autosomal recessive diseases that like 4 people on the planet have, and game the test. The curve is tight, and performance on the test has stopped reflecting ability to actually perform the jobs the test is gatekeeping for.