Building rapport is an art. It may happen that you are both are very similar to each other, and then the rapport will be there naturally. However, in most cases, you will have to build it if you want to get benefits from it. And the benefits are huge: building rapport is like making the other person like you. And if they like you, you can lead the conversation to any point you want.
The most fascinating benefit of building rapport is that from the point you have a strong connection, you can change the emotional state of the other person in any direction you want to. This means first you try to mimic the other person and match to their energy, and second, you go slowly toward the energy you want (for example, from negative to positive).