My $0.02 ($0.00 since we don't have pennies anymore): it's always helpful to have alternatives in mind to suggest in case of flight delays or cancellations. Passengers could be queuing up with dozens of ultimate destinations if the flight goes to a major connecting airport; the ticket agent can only hold so much in his or her head and to expect them to remember all possible alternate routings to each destination is unreasonable. You, on the other hand, only have one ultimate destination (well maybe 2 or 3 depending on the number of airports serving the location). Looking up possible alternate routings beforehand means you have suggestions for the ticket agent to look up rather than relying on their experience and the computer (their experience may not ever be a factor, since airlines change their schedules all the time and the rerouting they used last time may not work anymore).

Computers make it easier, but depending on how they were programmed they may not come up with absolutely everything.