Mulch prevents water and wind erosion and stabilizes soil temperatures (warmer in winter, cooler in summer under mulch).
It also reduces evaporation of moisture from the soil, reduces weed competition with landscape plants, and prevents damage to trees and shrubs caused by lawn mowers and string trimmers.
Mulch creates a better environment for many beneficial soil organisms. As it decomposes, it improves the soil.
It also reduces soil compaction, increasing infiltration of water and air into the soil.
This results in healthier tree and shrub roots to grow healthier trees and shrubs.
These benefits also help herbaceous perennials.