Method SetValue

Sets the control's value, updating both the displayed content and any stored representation the kind keeps.

public void SetValue(string value)

Check box — value is parsed as a boolean; the stored state and the painted symbol are both updated.
Drop-down list — value must match an entry's value or its display text, because Word does not allow anything else; the entry's display text becomes the content.
Combo box — a matching entry is used when there is one, and free text is accepted when there is not.
Date picker — value is parsed with InvariantCulture; both w:fullDate and the displayed text are updated. Use SetDate(DateTimeOffset, CultureInfo?) to avoid the parse.
Every other kind — the content becomes a single run carrying value.

Setting a value clears w:showingPlcHdr, which is what Word does the moment a control receives content.

ArgumentNullException: value is null.
ContentControlEditException: The value is not valid for the control's kind, or the preserved markup could not be rewritten.