You do not have to give but you have to wait 30 days to take another after you have taken something. It is a nice way to try new things or to give things away that are collecting dust.
Sometimes the person giving asks you have given in the past so many days or other little things. It's fun!
If you post wrong they will politely correct you and help out.
I was unsure of how it worked too, but maybe I will try it. I received a 15ml free from The Vapor Room for being a new customer. Not something I would have ordered. I can't get past the smell, let alone try to vape it. LOL so I know its just gonna sit there collecting dust. I am sure someone else would love it.