But not as important as the work that they do from day to day.
It isn't at all in line with end of KS1 assessment - which must be collated using lots of different kinds of evidence to give a fuller picture, not just a one-sided view. It's more evolved than giving a small child a narrow test, and gives much more information aout their attainment. See Building a Picture of What Children Can Do.
Year 6 also requires the reporting of teacher assessment, which has 50% weighting against the tests. So teacher assessment remains very important throughout the whole of a child's primary education, along with some testing. But it isn't the be all and end all, and nor should it be.
Most GCSEs have coursework attached to them, which can be likened to teacher assessment.
The few schools who only report a test result will use their wealth of teacher assessment alongside their test data to track underperformance, plan interventions, plan next steps in learning, etc. So why hide it from parents?
I understand that it isn't statutory to report any test data at all until Y6, and that teacher assessment must be reported in Y2 and Y6 - but if you are going to report anything at all to parents in the other years, why choose something that is such a small part of their learning and the tracking done at school?