Why did Buddha put so much emphasis on no-self? Why should we care if we have no-self or have a self. Ultimately it is of no help. I know any buddhist teacher will say that feelings or body are not-self, so you shouldn't get attached to it, but even if we do assume that we have no-self still we face the consequences of kamma. We just can't pretend that this body is not-self and cherish once it starts to decay. Everyone understands that no-self is truth but how can it help us remove our suffering? The only option I see is that you pretend that because of no-self your suffering has lessened, but does this pretending ever help? No,never.

In short, how does the theory of no-self help us in a real sense, not just in pretending sense. Why is this no-self so important to Buddha?
You're speaking of the self as if it is real. Only when you know it is not does the benefit of knowing otherwise become real. Being told the self is unreal will be of no help to you except as a clue. The idea is most definitely not to pretend anything.

You say 'Even if we do assume....' It would be an error to assume anything, and of no use at all. When you see beyond the self you wont be asking about the benefits but marveling at them. The theory is practically useless, the reality is all that counts. 

You say everyone understands that no-self is truth but this is far from being the case. Even you seem to be questioning it. I'd guess 1 in 500 would be an optimistic estimate of how may people think non-self is truth, and those who know are even thinner on the ground. Most people wouldn't know what you're talking about.   

The 'theory' of no-self is not designed to help us. It's not designed at all. It's just what is the case. It is not a theory for those who know but just the way things are.