Mulch can be used as you stated for aesthetic reasons around plants.  It depends on the plant and the soil whether mulch is necessary for the plants health. Some plants do struggle with soils that warm up quickly in the summer or soils that dry too quickly in the heat of the summer.  For these trees mulch can improve their vigor.  Of course mulch can be used to protect trees from damage from mowers and weed whips that tear the bark creating a wound that is usually irreversible.   Mulch may not be a necessity for every tree but it would be a benefit in most situations.  It can protect a young tree during times of  drought (2007 and 2012 were significant droughts across Michigan).  Mulch prevents quick loss of moisture during dry, hot periods especially important for young trees.  Plants that grow naturally as understory plants in a forest like flowering dogwoods and redbuds are use to the cooler forest soil and would benefit from a few inches of mulch spread evenly over their roots. Of course watering is a key factor to the health of plants during especially hot dry periods.  Mulch does not replace the need to water but can reduce loss of moisture when nature is not providing adequate rainfall.  The one area I would really encourage mulching is where large riding mowers are used near trees, such as housing complexes or large commercial landscapes.  These are sites where mulching (if done properly) can prevent unnecessary damage as equipment moves between trees.  Mulch should never be mounded in a cone up against the trunk but spread evenly at a depth of 2 to 3 inches. See the following article for more information: https://pender.ces.ncsu.edu/2012/10/can-too-much-mulch-killl-plants/