Certification gives you peace of mind to know you are receiving exactly what you paid for. Large diamonds are an expensive purchase and any very minor change in quality can often mean a big change in the price.
Previous to international certification a person would just have to trust in the reputation, and their judgement of the jeweller the know the quality of the diamond that they were buying. Now with internationally accredited laboratory certification the quality of a diamond is precisely measured and unquestionable.
Taking the need to trust the word of a sales consultant out of the buying decision. Also available now is laser inscription when the diamond certificate number is engraved into the girdle of the diamond which can be easily under magnification.
By making sure your diamond has certification you have total assurance of the quality you are receiving and when deciding on your purchase you can get a true comparison between the stones you are selecting.