shortening and combining Prayer

What is the reason for shortening the Prayers—is it difficulties encountered or travel itself?

Dear scholars, As-Salamu `alaykum. I understand that when traveling, the rules for Prayers for travelers only take effect after you leave your place of residency. However, what is the rule if you know that, for example, you are leaving your town after Zhuhr Prayer and will not arrive at your destination until Maghrib. Can you pray and combine your Zhuhr and `Asr while in your city at the time of Zhuhr? If not, what is the right thing to do? Jazakum Allah khayran.

As you know, while travelling, Muslims are allowed to shorten and combine prayers, like performing Zuhr and `Asr prayers together. My question is, how do we know the distance that permits such dispensation? Do we need to make the intention of delaying, say for instance, Zuhr or Maghrib during the due time of anyone of them then perform either Zuhr or Maghrib with `Asr or `Isha? I say “delay” because someone has told me that one is allowed to delay prayer while travelling, just for the sake of travelling.