Is it important to say 'thank you'?Why?

Because saying thank you acknowledges our dependence on others

To acknowledge someone for doing something for you, it's the right thing to do. And while saying thank you doesn’t necessarily open doors, it may just leave them cracked wider for your next transaction. Thank you is also a small payment forward: in those cases where you may never see one another ever again, its echo leaves the door open just a little more for those behind you making the same request.