From what I saw of the SAT I formed the impression that its sole purpose was to reward kids who happened to be good at that style of test taking. The format was frankly weird and a race against time more than a display of depth of knowledge.
It seemed to me to be a test focussed on the glory of itself
Well, there are perhaps several purposes for the SAT. First, the College Board earns over a $1 billion, and the CEO earns over $2 million a year, even though it's ostensibly a non-profit. Then, there's the SAT test preparation industry which collects tens of billions each year. Money is definitely a key purpose of the SAT, at least for some people.
The key reason for the existence of the SAT is that it's perhaps the least easy of all admissions criteria to manipulate. GPA grading scales and criteria are highly variable across different high schools, and even at the same high school. The GPA for one student is not obviously comparable to the GPA for another student, and yet, colleges make the comparison even though they know that the comparison for any two students is tenuous. The correlation between high school GPA and college success may be moderate on a population level, but the correlation between any two arbitrarily chosen high school students can range for extremely weak to extremely strong. Essays are the new GPA, but essays are far worse than GPAs, as they are extremely easy to fake and manipulate with absolutely no way for any university to discern authorship or authenticity or to institute review consistency.
This is also the underlying reason for criticisms of the SAT, that it's "unforgiving" in not allowing specific biases (for minorities, poor people, less intelligent rich people, artistic/sporty but less academic people, etc.) to be indirectly favored. Instead, specific compensation in the form of affirmative action, legacies, artistic/sports scholarships, etc. are needed, and those types of compensation are being legally or otherwise attacked.