The techniques behind handpairing butterflies is probably less known than other techniques of rearing butterflies. The value of handpairing butterflies makes sense when rearing multiple generations of one taxon from one location is desired.
For example, let's say you only collect a few swallowtail larvae from a location; but would like more individuals. By rearing through the larvae you have to pupae, emerging them, and then handpairing a female to a male, you give yourself an opportunity to get many more ova; and hence adults from the same location.
Below is a video demonstration where Wayne Whaley is handpairing eastern black swallowtails.