Oh, it's so nice to have people ask. Because I'm in Secondary, I deal with KS2 SATs data.
Firstly, the SATs have changed to reflect changes in the KS2 National Curriculum. Instead of just levels, we receive scaled scores instead, which enable greater differentiation between students.
They're divided into Maths, Grammar/Punctuation/Spelling and Reading for the 80-120 scaled scores (90-110 is the mid range, 100 is the baseline). 80-90 is lower ability, 110+ higher, then there are two teacher assessed components for Science and Writing, which are assessed as working at greater depth, meeting the standard, working towards the standard and then a whole bunch of specifics for not being in the 'normal' ranges for their age.
From the point of view of a secondary, they're useful in identifying children who might not have been focused on in terms of having additional needs at primary (although they are limited by being designed to provide a measure of teaching over the key stage, not ability, so it is possible for a kid to have low scores as a result of an inadequate school/poor teachers/no regular teacher/etc - or of low attendance due to disability/illness/family pressures/being new to the country).
Combining those with CAT4s give a really good picture, though, particularly when you're looking at a spreadsheet where somebody has lower or average SATs scores but their CAT4s make it clear that their Quantitative, Verbal, Non-Verbal and Spatial skills are very high or they have high ability in some areas (that's where we refer to a 'spiky profile' - it doesn't just identify SEND in the way most people would imagine, it also shows up potentially exceptional abilities). CAT4s are also less dependent upon physical ability - as a child who struggles to hold a pen or tires easily when writing due to hEDS could find it much less physically taxing to take a computer based CAT4 - and can also help those with ADHD or other conditions affecting attention span to perform better. Computer based CAT4s are also graduated in difficulty depending upon previous answers, which makes it less likely for a child to panic or be discouraged from the moment they start the test.
Schools may also administer a Reading test to accompany the CAT4s. This also provides valuable data and can highlight if somebody has a significant difference between attainment and ability.
In short, with all that lovely data, we can see if somebody's been let down in primary and take steps to address it from day one - and there's greater accountability because if we screw up, that data is sitting there to be compared with the outcomes of every data drop and there will be very pointed questions asked. And, on top of all that, we can see if there's maybe something going on with the child themselves - because if somebody who has consistently been assessed as being of a particular ability suddenly has results that don't match it but others in their class/year group do, then it's very, very obvious to us spreadsheet wranglers and all who endure our missives narrative on the analysis. Which could be even more important in the long run, as it crosses over into welfare, wellbeing and safeguarding.
I luvs data, me.