Word order is used for simple and frequently used features of the grammar. The grammar has to allow combining any message with more information. The main bulk of words mean either an object, an attribute, an action or an effect. The language must have tools to make identification of objects efficient. The language must have a way to signify repeated reference to the same object, and make it cheap.
Languages are far from optimum.
Any language that is used for communication by humans must have these features, independently on any innate rules. Therefore, finding these rule does not support theirs innateness. Other, more specific rules, are more difficult to predict directly from the communicative role of language, because our understanding of language and human cognitive performance is not good enough, but they seems to be plausible. This is an example of using word order for combining the meaning of words to the complete phrase. Fixed location in different types of phrases is presumably less confusing than different word order for each kind of phrase.