If, for example, your XML looks like <Foo>Bar</Foo> then “Bar” is actually considered a separate node: an XmlText node (sub-classed from XmlNode). The Value property of that XmlText node would be “Bar”.
“Foo” is considered to be an XmlElement (also sub-classed from XmlNode). XmlNode.Value returns different things based on the type of node it is. See this table which shows that Value always returns null for Element nodes.
The InnerText of the Foo node returns “Bar” because it concatenates the values of its children (in this case, only the one XmlText node).