I invest some money in this game. Why? Because I enjoy playing it and want to get more fun out of it. I'm sure you'd enjoy it too if you bought some tokens. Of course you don't have to buy them because people succeed without buying scouts/huge amount of tokens but everyone is different.

I mainly get my source of tokens from completing offers which involves paying for stuff, so I get something I like in real life and get a little token reward as a bonus.