Admissions committees look at a student's entire transcript to get an understanding of their academic abilities. Why is it not sufficient to judge a student's academics based on GPA alone?
The problem with GPA as a measure is that it just mixes together a bunch of different classes indiscriminately. Consider these three different students:
- This student is pretty steady, but not great, and tends to get an even mix of As and Bs across all of their classes, obtaining a 3.5 GPA.
- This student always takes the class with the easiest grading policy whenever they can, and tends to get mostly As in the "easy" classes. When they cannot evade a "hard" class, they get mostly Cs, resulting in a 3.5 GPA.
- This student is very strong and gets mostly As, but there was one semester when family problems disrupted their life, and missed assignments caused them to get Bs and two Fs, also ending up with a 3.5 GPA.
As you can see, even though all of these students get exactly the same GPA, they are very different. Student #2 is clearly far weaker than Student #1, who is in turn weaker than Student #3, whose GPA has been strongly affected by the one problem area.
If we can see which classes they took and what grades they got on those classes, we still don't have the full story, but can get much closer to an appropriate evaluation of each student's actual strengths.
In addition to jakebeal's answer-
Grad schools are looking for people who will make good researchers. We figured out long ago that great students (who have good GPAs) are not necessarily great researchers. Similarly, some students with middling GPAs can be great researchers, even though they didn't score top-tier grades.
A graduate admissions board is probably far more interested in a 3.5 student with a a research project or two rather than a 4.0 student with no extra-curriculars. (In fairness, I do know of a few people who take the opposite viewpoint- they get enough 3.9's and 4.0's applying to their programs that they don't see any value in considering other students. However, those people do not serve on admissions committees.)
For a more decision-centric answer, GPA by itself can only really be used to exclude people. You might discard the bottom 50% of applications by GPA, or you might establish a hard cutoff by saying you must have a 3.0 or higher. It doesn't give you good reasons to include people, and when committees make these decisions what they really want to do is find great reasons to include applicants.
Consider how these three statements would sound to someone who is trying to figure out who will make a good researcher:
"This person has a 4.0 GPA."
"This person was an excellent student."
"This person has some research experience and would like to understand better how to build city-scale sensor networks."
There are two major reasons, both of which boil down to "Context".
- Graduate school is not "Undergrad+". There are few if any distribution requirements, and your coursework will follow a pretty specific subject. In contrast, your GPA is an amalgamation of all the classes you have ever taken. To use myself as an example, some major components of my GPA stemmed from 4 semesters of German, a feminist art history class on the middle ages (which was amazing) and a class on Grimm's Fairy Tales. As importantly, a number of classes didn't count toward my calculated GPA because they were taken while I was studying abroad, and thus the credits simply transferred. Most of these classes were in Microbiology. Assume I barely passed all of those. If I was applying to a microbiology program, do you think they'd prefer to see those classes broken out individually, or a composite score that's heavily influenced by being able to give reasons why the Bayeux Tapestry is more properly an embroidery, and why the Saxons in said embroidery are depicted wielding axes?
- It lets the admissions committee actually look at you as a whole person. Did you take some classes outside your major that would never the less be useful? How well did you do in them? Did you take classically challenging classes (Organic Chemistry) that might explain why your GPA took a hit, or were you simply a middling student?