what are the iba evidence rules?

Formally known as IBA Rules on the Taking of Evidence in International Arbitration, they are a set of evidentiary rules formulated by the International Bar Association (IBA) that parties may, but do not have to, adopt for use in their arbitration. The parties are free to adopt the rules in whole or in part and to agree on modifications.

why do we need evidence rules when we already chose to conduct our arbitration under the rules of an arbitration organization or institution or under the uncitral rules?

You do not "need" the evidence rules, but they may be helpful because "arbitration rules" generally do not include evidence rules, so that without the IBA Evidence Rules, often nothing has yet been agreed upon in this regard.

do the rules give an advantage to lawyers from a particular country or legal culture?

No, they do not. The IBA Evidence Rules were formulated after careful deliberation and public consultation with a large number of lawyers from around the world and attempt to strike a balance that appeals to lawyers and parties around the world, irrespective of their particular nationality or legal background. The Rules have recently been updated.