Hi JustaDadHere
Agree that in your DS's case - as he's off to an independent school, possibly having taken a competitive exam to enter - SATs are not desperately important. But it will be good practice - because odds are he'll be tested a lot in those first weeks of Y7 at his new indpendent (private) secondary school.
However SATs are only one of many measures the school will take into account in relation to predicting his expected results at GCSE/ A-LEVEL.
I am only a parent and frankly am only really familiar with the state sector - but KS2 SATs are the base-line score against which progress in secondary school is measured by OFSTED. Rumour has it Gove is about to insist that independent sector schools are also subjected to OFSTED inspections.
For example DofE have this table showing how KS2 SATs relates to GCSE grades at end of Key Stage 4 (Year 11): www.education.gov.uk/schools/performance/archive/schools_10/s11.shtml - the first green squares in each row are basically the lowest expected achievement (but the underlying statistics suggest that achieving higher - further along on GCSE grades - becomes less and less likely the lower your KS2 SATs result).
So in essence scoring NC L6 at KS2 SATs means it is highly likely your child will achieve A possibly A* at GCSE (end of KS4). It is of course no guarantee but it does suggest the direction of travel, if you like.
Now again, it may not seem hugely important, but parents considering your DC's primary school for their own children will look at his year's KS2 results next year (this year's KS2 2014 SAT's results will be released Dec 2014). So people considering his school will be impressed to see that the school is capable of getting pupils to NC L6 (o.k. maybe well supported pupils at the school, whose parents do quite a bit of enhancement through 11+ prep/ tutoring/ etc...).
So if your DS liked his school, enjoyed going there and learned a lot whilst he was there, doing well on the SATs helps to convince other families that this is a good school. It helps to make his school/ teachers look good to prospective parents/ to OFSTED.
Obviously one student alone can't make a huge difference, but as a group their Y6 results as a whole will be looked at by prospective parents and every L6 or L5, as well as high % attaining NC L4 or better in English/ Maths combine will all help parents to decide whether the school is right for them or not.
HTH