The sieving of raw eggs is particularly effective when poaching or frying. The whites thin with age and cause untidy poached eggs with loose threads, and thin, spread out ones when frying. This process begins very quickly and will be evident in all but the most freshly laid eggs, thus sieving them will purge the eggs of the outer, thinner layer, resulting in neat, and comparatively plump, cooked whites.
It is best to use a sieve with a fine mesh, otherwise you could lose more of the white than necessary. You only need to leave the sieve over a jug or bowl for a few minutes before the watery parts are drained and then cook as normal. You can see the difference between the two fried eggs, sieved and unsieved, on the right. Both eggs were the same size and age, from the same box, and in each case, the yolks are not yet set as these are for illustrative purposes only.