Legally the position in the UK is that PARENTS, not the state are responsible for a child's education.
This exempts the state from being sued when schools fail children, and we all know that sadly too many kids are being failed. A quick glance over at the SN boards will show that a universal adequate education provided by the state is a distant dream for many families.
SATS are just a way of ensuring that schools are fulfilling their responsibilities to parents, despite the spin & pressure schools put on it.
In some countries such as Germany home ed is illegal, that also means that the German state has accepted some legal liabilities that the UK government would run a mile from at this time.
Home education is simply one option amongst many in which some parents choose to exercise that right in a proactive rather than passive manner. Others include being a governor or active member of a pta, going private, volunteering at their child's school etc, etc.
My son is currently following a route already trodden by thousands of US, Aus, and Canadian schools - he attends school online. Online schools will I think eventually be a major disrupter to the UK education system, in ways we can't yet predict. As it is they are reaching a whole cohort of children previously unable to access a conventional mainstream education. Just as the world of work is changing, so the world of education needs to adapt for the modern world.
Alternative curriculum's also exist within the conventional world of school, state and private schools are choosing to offer the IB, the American High School Cert and the igcse as alternative models to the state sanctioned GCSE courses.