Social housing is the name given to public sector council houses or homes provided by housing associations or registered social landlords. Social housing came about because housing is a basic human need. Everyone needs a home - but most people can't afford one.
Homes are expensive.
But if everyone pools their resources, everyone can have a home. This is a model of collective provision. Successive governments have prefered an alternative model of social housing. The early history of social housing is a fight between these two models. These two models of social housing are still argued over today.