Benefit of a 1.25" line through a 1" supply?

I'm re-working the failing irrigation around my home. The main line around the property is 1.25" PVC with 1" valves and laterals. However, it all comes through a 1" shutoff valve and backflow preventer at the meter followed by a 1" to 1.25" PVC adapter.

I'm new to the world of irrigation and am not understanding the benefit of using a larger main line than the supply given the added cost of the larger PVC. Are there any? There are moderate (~6-8 ft) elevation changes around the property, if that matters, with the water meter being near the highest point. Thanks for any insight!