Our Sunni fellows get surprised when shia pray Dhuar and Asar together and Maghrib and Esha'a together. But they dont know it is all proven the Sunnah of the holy Prophet (pbuh) which mentioned in Sahih Muslim.
Please find the narrations.
Another Narration
These narrations are very well self explanatory, that holy Prophet combined two prayers without any fear or any emergency.
The narrations says itself, it is to keep away people from unnecessary hardship. We can experience this in our daily busy life.