It's just the expression in English "it's the least I can do" is a polite of saying don't worry about it, it was nothing.
I disagree.
It sends a message. If a person on the street asked me for directions and thanked me - I won't answer with "thats the least I can do" ill answer with "sure". But if it's a person that I feel like that's been helpful to me yet he thanked for me for something...that's the moment to say "that's the least I can do"...so I won't just throw this phrase around for no good reason.