When trying to use autocomplete on a XPath constraint or expression, it does a "starts with" search.
Sometimes you don't remember the first characters of an attribute/function/etc, so with the autocomplete doing a "starts with" search you wouldn't find what you want.
E.g.: You want to add a XPath constraint for the "status" attribute of the Course entity. You start typing "[st", click ctrl+space and the suggestions don't show the correct attribute. This is because the attribute is actually called "CourseStatus". If the autocomplete did a "contains" search it would've showed what you wanted to find.