Mulch can reduce exposed soil, moderate soil temperature and reduce evaporation while suppressing many weeds.
Don't bury trunks
Keep mulch away from tree trunks and root crowns. Deep mulch piled against bark can create moisture and decay problems.
Match mulch to the planting
Mulch type and depth should fit the plant community and site. Some California native landscapes require more nuanced surface management than simply applying a thick layer everywhere.