Socialisation of young people is vital for social and emotional development. Preschools allow children to mix and develop in an environment designed to promote their development. As my child is an 'only', I was keen from the first few months that he would encounter other children and would be managed at these times by people trained to do the job. I'm a high school teacher, trained to deal with teenagers. I was conscious that my child would benefit from the professionalism and expertise that a good preschool can offer.
Siimilarly, I regularly set up and make use of playdates. my child is a very independent soul, so experience of playing with others allows the teaching and learning of sharing, turn taking, tolerance of others, etc.
Homeschooling of young people due to rural/environmental/medical issues is common here in NZ. However, most parents tend to use the Correspondence School as a curriculum base, resource and provider of distance teaching resources. Many of the homeschooled children I encounter do well academically as a result of this approach, but many more have difficulties in relating to their peers.
Social skills need to be learnt early, often and continuously. I see them as more important than academic hothousing.