Mycorrhizae are especially important in the regions where the soil is deprived of water and some nutrients (a desert typically).
Mycorrhizae also allow the plant to acquire additional nutrients and moisture, which is essential for phosphorus to be up-taken.
As a result, it is the presence of Mycorrhizae that causes plants to be less prone to water stress.
In addition to allowing the fungal threads to draw nutrition and water closer to plants, it also stores them which is to be used when the precipitation is low and temperatures high. Hence, it is an important aspect.