Why do ICO's give away free tokens and what is the region to doing this ?

It brings people to the project that otherwise would not have owned or heard about it. It could lead to token price appreciation, since people value a token they own higher than a token they don't own. This is called the endowment effect.

Airdrops create a community/network of people who own the tokens. If you would list the token distribution after an ICO in a pie graph, a large part of the pie is still owned by the Dev's or project. Another large part is owned by people who joined a pre-sale. And a reasonable part is owned by people who invested in the ICO. An airdrop adds an extra slice to the pie and that slice will have the most people in it.
Looking​ ​forward,​ ​possible​ crypto ​airdrop​ ​developments​ ​could​ ​be​ ​projects​ ​starting​ ​to​ ​gift​ ​tokens​ ​to​ ​users​ ​who hold​ ​tokens​ ​in​ ​projects​ ​that​ ​overlap​ ​or​ ​have​ ​some​ ​synergy​ ​with​ ​their​ ​own.​ ​We​ ​could​ ​also​ ​see​ ​projects encouraging​ ​other​ ​projects​ ​to​ ​airdrop​ ​to​ ​their​ ​token​ ​owners​ ​and​ ​receiving​ ​some​ ​benefit​ ​in​ ​return. Like we already see happening at the Ethereum, Waves, NXT & Komodo platforms.