> the main purpose of the thing

It is not. But France was very good at ring-fencing their interests very early on, resulting in a somewhat-outsized weight of agricultural policy over the Union budget (since it was, back then, almost non-existent in other areas). After the Eastern expansion, it has become very difficult to change the approach (which is, overall, fundamentally successful - yes, there are issues, but nothing is perfect). Pre-brexit, the UK government would be the only one willing to grandstand on reforming the policy, mostly for reasons of internal propaganda; now it's basically in no-one's interest to touch it.