Most .co.uk's etc normally forward you to a .com if they own both domains (Im tempted to buy my .co.uk but I bet someone beats me to it lol)
It doesnt really increase profit or anything its just so if people type the wrong .*what ever* it goes to the correct one to make sure that no one else can sap off of their popularity/numbers.
EDIT: Some can be manipulated to showing a single pixle frame to allow for its own Page Ranking so a site could have two sets of Page Rank (not 100% sure on that but I have checked one or two which use a 1pxl frame and it had a higher
PR then the .com as its linked to the domain)