Do we need to learn idioms?

I would say there is no need for a foreign speaker to use foreign idioms. They'll often get them wrong anyway and just confuse the conversation. Rather they can introduce their own native idioms into their speech, to give it colour and character.

However, a foreigner listening to, or reading English, must know and understand the meaning of the new language's idioms if he wants to understand what he is listening to or reading. Pick up any newspaper, book, movie or TV show and you will find it thick with idiomatic phrases. It is one of the reasons that computers do so badly translating: they can't understand idiomatic speech and only translate literally.

So, yes, you do need to learn idioms.