do you thank is it important to say the word thank you?YES OR NO ?WHY

yes because it is important to thank poeple who make a service
and when it is you. I think you appreciate when the people say thank you to you

I think that its important to say thank you because it shows the person around you that did an action towards and you appreciate them doing that. 
It is important to show how grateful you are. 
It is a mark of politeness. 
good luck