Other than the obvious reason to save money for yourself, there are some very good reasons why to back to back ticket (all honorable?). I am referring to back to back ticketing as buying one ticket from A to B over a weekend and the other from B to A over a weekend in order to get discount fares. The legality of this is in some question (see other thread on antitrust), although it appears if you do it on two unrelated airlines, this may not be true back to back ticketing. Why do it (for honorable reasons?)

Charities don't have lots of money but their staff and volunteers have to fly, sometimes to meet with decision makers who don't work on weekends, i.e. no cheap fares. To fly from the west coast to the east coast on a same day or next day mid-week return will cost me $2100 CAD--one may not mind this for business where one can bill a client but for charities, we should be more creative and cheap! If you fly mid-week on each part of the back to back tickets, you can fly for $300 return for each = $600 in total. If you plan it right, you not only use the first part of each back to back ticket, you can also use the return coupons of each ticket to create another trip for the charity. You can fly twice for less than a third of what it costs to fly once full fare. This is big money for charities!

There are some disadvantages -- usually they require advance booking (14, 7 or 3 day) and changing the return to a different date requires a change fee (you can usually fly standby at a different time on the same day for no additional charge)--but it is still cheaper than full fare.

Many charities hold meetings over weekends so they can get the benefit of over weekend discount fares but this is not always possible for every meeting.

I hope that is an honorable (and practical) reason for back to back?