Water reacts with more substances than any other compound. It reacts physically with several compounds to add to their crystal structure. Compounds like copper and magnesium sulfate are two examples of many compounds that almost always found in nature with water molecules physically attached to their crytal structure. Some of these compounds, have water as such an important part of their structure that they will even extract all available moisture from the air. These compound are natural dehumidifiers, dependent on water to complete their structure.
Water also reacts chemically with many substances, creating new substances or compounds.
Dissolve Hydrogen Chloride gas in water and the result is a powerful acid, Hydrochloric Acid. These are but two of many examples of water combining with another compound to create a new substance. In some cases water's presence in abundance explains why some substances are not usually found. For example pure forms of two abundant and common elements on earth, almost never are found in nature. Sodium and Potasium, both metals in their pure form, are almost never found in nature because they react instantly and explosively when brought into contact with water.