What are the benefits to send 18mth to a playground ? For example what kind of skill will he learn from attending playground that cannot learn staying at home ?

Yes i think you meant playgroup. I believe its just the social skills, learning to share toys, playing together. That said, there are many skills that you can impart at home too ! See link here. http://www.friso.com.sg/learning-and-experiences/emotional-strength/essential-life-skills-your-preschooler-can-learn-at-home.html