Sparrow has both rear dew claws, as she is a working cocker she will never enter a show ring, but I have been advised to have them removed because they may snag when she does agility.
There is a lot of debate as to why some dogs are born with rear dew claws, no one seems to quite know their purpose. They are generally unattached to the main structure of the foot except by tissue, so are not of particular use. Though some argue if the rear dew claws are in a position where they touch the ground the dog may use them for balance and removing them could cause them to adjust their running style.
I have read they are a vestigial toe from the days of early dogs or wolves who needed to climb terrain regularly.