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 ?

If you mean playgroup, attending one can hone his/her social skills. It will teach your lo how to deal with other in terms of sharing, behaving and others and playgroup teachers also teach songs, dances, colours, number etc. IN short, they can make learning fun. It can also be expensive though. But you can invite some mummy friends to your home and your kids can play together and you can take turns in teaching the kids songs and dances. Now, this does not cost anything, except for coffee and snacks perhaps :-)