Take a case where a party thinks it doesn't have the opportunity to win both seats, and let's say it anticipates coming in third place. In that case, they would probably deem it more reasonable to allocate all three resources to one candidate. The one candidate gives the party the possibility of winning that second seat when that decision-making process comes into play, but it also allows the party to get votes that count toward the regional result for allocating seats to each party on a proportional basis.
Running the second candidate is only necessary if the party anticipates it can win the first seat in the district. That would maybe show strength for the party when they can nominate that extra candidate, but otherwise it's not necessary.