it's a method used to optimise the output of genetic circuits, functions of proteins, metabolic processes etc.
we need a large sample group to improve the chances that a mutant would have a desirable property. we then select for this desirable property and grow this colony, to have bacteria with the desired property.
say we have an antibody and a substrate that doesn't fit too well to the binding site of the antibody. we perform directed mutation on the antibody and hope that mutation would occur and the substrate binding site of the antibody will change to fit the substrate better, and we'll have the perfect antibody. the problem with this application is that antibodies don't exist in bacteria.