The market value of your property is simply the probable price that it would sell for in an arm's length transaction between a willing buyer and a willing seller. In Michigan, market value is defined as "True Cash Value."
Determining a property's value is the Assessor's job, and is required for every piece of property, no matter how large or small.

Also, each year the Assessor has to do the whole thing all over again, because the market value of almost everything changes from one year to the next.

The Assessor is required by state law to assess at 50% of true cash value all assessable property, as of December 31. This includes homes, factories, commercial properties, vacant and personal property (machinery and equipment for commercial and industrial properties.)

WHY ASSESSMENTS GO UP WHEN A PROPERTY HASN'T CHANGED OWNERSHIP

Since assessments must be set by market value, changing real estate values in the community will be reflected in the assessments. Market value is a product of the prices paid for property. As prices increase/decrease, so does market value.

All properties do not change in value to the same degree. Many factors influence values. Those properties with water or scenic views, for example, may well increase more rapidly than others.