Mulching your plants offers so many benefits, the question should be, "Why would you not mulch?".

Suppresses weed growth
Retains moisture in the soil for plants
Insulates the soil to moderate temperature extemes (heat and cold)
Replenishes nutrients back into the soil
Increases microbial activity in the soil, helping plants
Reduces or eliminates soil compaction
Reduces or eliminates erosion
Can be used to mitigate compaction around tree root zones in pre-construction areas
Adds beauty to your landscape