SATS are a waste of time and effort, and only show how a child is doing on a given day. It is much better for continual formative and summative assessments to be done by teachers who know the students. I spent 3 years persuading a lad who had less than average SATS at year 6 and year 9 that he could achieve, as he pigeonholed himself as thick. He is a very bright lad, and achieved excellent GCSE grades and is now doing A levels with a view to reading law later. He spent all that time with no confidence in his own ability because of the SATS.
SATS tell you squat really, and they don't tell you how a student is going to do in your subject at GCSE, as they don't test all subjects. Internal exams at the end of each academic year are far more beneficial (I know the marking is a drag), as the students then get used to the exam set up, and having to revise for a range of subjects, which is what they will have to do at GCSE anyway.
DS goes to an International school near Brussels where they have just dropped KS2 SATS, having done nothing but SATS practice from October half term up to the test. So much for a breadth of education in Year 6! They then announced that this was the last year they were doing them! Waste of time, money and effort, and not marked externally either. They don't do KS3 SATS and assess internally, which is better, as teachers (being the professional people that we are), do actually know how out students are doing.
SATS stress the students unnecessarily as the school has to get results. As previously said, the SATS test the teachers ability to teach to the test and the exam skills needed to attain the top levels, not the students ability to apply information to the questions set.