What exactly is the idea of bediaks chometz nowadays as we know we certainty that there is no chometz left in the house by then? Is the house not classified as a mokom she`eyn machnissin boi chometz? How can one make a Berochoh?
This issue is addressed by the Shaarei Teshuva 433:2. He explains that based on our earlier days [and weeks!] of cleaning, on the night of bedikas chametz we go over the house in a more superficial way to assure that everything was cleaned properly. Generally there is no problem of a “makom shein machnisim bo chametz” as until the erev Pesach there is still chametz in and around the house, especially when there are children around. Hence there is still a need for a bedika, and to make sure everything was cleaned properly and nothing was overlooked. In addition there are often places that were not checked in the crevices with a candle or flashlight, and this is done on the night of bedikas chametz.
See also Maharsham Daas Tora 433:2, Nitei Gavriel Vol. 1 Chapter 23, footnote 7.