Why is supply side policy used a lot in modern economies?

Supply Side policy is basically anything that improves the long run growth of an economy. By that, we mean, anything that can help it grow in the long run sustainably, thereby avoiding 'bad' inflationary pressure usually experienced by a rapidly growing economy. Therefore, it is used alot by modern economy's, simply to improve on long run trend growth rate, so that these economy's are able to sustain a high level of growth. The 1970's was when Margaret Thatcher really introduced substantial supply side policies such as privatisation, a completely new tax system, that one can easily led to rapid growth in years to growth. Supply Side reform continued leading to upswing periods of growth, most notably, the 'NICE' period in 2003 (Non inflationary Continuous Growth). Education is a particuarly important SSP, and as you are probably aware, is being heavily reformed in the UK. Why? So that we have brighter students, that are more skilled and thus employable, therefore reducing the unemployment rate and having a skilled labour force that can support an ever growing population.