Why not?!

A number of places in the world seem to have a saying or terms of phrase all the locals use and the tourists love.  From 

"same, same but different"

"you like a genuine copy"

"see y'all all later now"

"that's like soooo"

"G'day mate"

"top of the morning to you"

"ciao ciao"

"sweet as bro"

"sorted"

"bosh" 

In India, it's "Why not" said as a statement/question combined, "Done" and "As you like".... 

Question - Please may I have a cup of chai?

Answer - Why not?!  Done

Question - Shall we leave now?
Answer - As you like...