If they can, this person is a keeper.
Communication will be tried and tested
Communication is essential to the health of any relationship. And nowhere will these skills be tested quite like while traveling together. If you can talk through disagreements while far away from what you know and your daily routine, you are on solid footing as a couple.
Traveling together forces you to either accept and work within the context of another person's limitations and expectations, or come to terms with being unable to handle them.
The sex is better if you can travel together
If you're compatible travelers, you're going to have better sex. Think about it: if you just had a fabulous day sightseeing and didn't wind up in a huge fight, aren't you going to want to get it in once you're back at the hotel?