Hi Sparkling:
Although I totally respect your viewpoint that your height/ weight is your affair....
just being devil's advocate:
Would it help the government to forward plan health services if they understand that obesity is on the increase? (there's been a lot on the BBC that actually research is now demonstrating that childhood obesity isn't running riot - and it is in fact gathering metric data on children that is overturning this view - e.g.; www.bbc.co.uk/news/health-25331997).
This could save the government millions in terms of having to plan to cope with side effects of obesity as adults (obese children typically becoming obese adults) - diabetes, heart disease, etc...
Would it help the government to better track 'average' growth? This is a real issue because the growth charts are largely based on possibly out of date data. (e.g. 0 - 4 growth charts based on data collected in 1980s - www.rcpch.ac.uk/sites/default/files/asset_library/Research/Growth%20Charts/Education%20Materials%2001092010/FS1%20What%20are%20growth%20chartsA.pdf
Knowing that children are getting taller or heavier has some planning implications for the state:
For example...
school furniture
school clothing
sporting equipment size ranges
Future classrooms may need to provide more of a range of seating options - not just those ridiculously small red plastic chairs.
Finally identifying onset of puberty is becoming more of an issue. For girls especially, puberty brings some pretty big changes and risks - educating them about their body changes is important and is a legislated requirement for schools. If more and more girls are starting menstruation in primary schools - schools need to be aware of this possibility, need to prepare children (and parents) for this and need to have things in place should the unexpected happen.
I hear it all the time - Children are growing up so fast these days - but they quite literally are - and adults who care for them (teachers, health professionals and indeed their parents) can benefit from better understanding what is typical development, when puberty might occur and what levels of weight/ height are a concern. Of course there will always be healthy, small, petite children - but small size & weight can signify other things - and personally I'd like to be in a society where someone is concerned enough just to gently check into things and ensure all is o.k.
Agreeing to have your child measured may seem an intrusion - but it could identify an underlying problem (e.g. www.rcpch.ac.uk/system/files/protected/page/2-18%20Fact_sheet_v8.pdf
Information on the reason behind the national child measurement programme is here: www.hscic.gov.uk/ncmp
Personally I have a string bean and a pear bod - I think they're both beautiful girls and want both of their heights & weights reflected in 'average growth' data - so that other string beans and pear bods don't feel odd or strange or weird about their bodies, but truly see it as within the range of normal.