Sahih al-Bukhari 1108

The Shariah of our Prophet Muhammad (may Allah's peace and blessings be upon him) is distinguished from the other divinely-revealed religions by its tolerance, ease, and the removal or alleviation of all aspects of difficulty and hardships from those competent for religious duties. One of the alleviating aspects is the combining of two prayers that occur within the same range of time.

In principle, these prayers should be prayed each in their own specified time. However, it was the custom of the Prophet (may Allah's peace and blessings be upon him) when he went on a journey to combine the Zhuhr and ‘Asr prayers, either praying them in the time of the Zhuhr prayer or in the time of the ‘Asr prayer. He would also combine the Maghrib and ‘Ishā' prayers, either praying them during the time of the Maghrib prayer or the time of the ‘Ishā' prayer.

He would take into consideration what was easier for him and the people who were accompanying him on his journey. So his journey was a reason for combining the prayers during the time of one of them, because the time becomes the time for both prayers. Travel involves a lot of hardship, either in stopping or in proceeding with the journey, so the concession of combining the prayers was only legislated to make matters easy during travel.