Because you can not takeoff planning to land overweight. Say your max TO weight today is 178,00 pounds but your max landing weight at your destination is 154,00. If your fuel burn is only 12,000 pounds to the destination you are limited to 166,000 even though the airplane is capable of departing at 178,000. I've never seen the case but you could in theory have a case where the alternate landing weight is further limited to say 148,000 but the burn to the alternate is only 4,000 pounds. If you departed above 164,000 you would be overweight at your alternate.