I did my SATs 20 years ago. Think they were a fairly new concept in the 90s(?) They were fine, scored well, no pressure really was bright child and did extended papers too.
Was described as a way to know where to level kids, results were used by secondary school to decided if you'd be in top-mid-low set for maths, sci, eng.
Obv longer term it has no affect on earnings potential, not put on cv or uni application, so minor benefits had as the test taker.
Nowadays, they seem to be a HUGE deal. Masses of pressure, painted as important! All for schools benefit. Not for kids well being. On that basis, WIBU to ask/decline for my child to sit the tests?