I understand the need for an array type in XPath 3.1 as they're fundamental to JSON. And yes I understand you can create a literal map() in an XPath query.
But is there a way XML or JSON can be structured where a query would naturally return a map on an XPath query against the underlying document? Or does it exist solely for the case where converting results into a map to then operate on is of benefit?
On
"is there a way [..] JSON can be structured where a query would naturally return a map?": anything in JSON being an "object"
https://www.json.org/json-en.html: "An object is an unordered set of name/value pairs. An object begins with {left brace and ends with }right brace")
maps (pun intended) to an XDM map.
So in JSON both arrays and objects are fundamental and in the XDM you can represent a JSON array as an XDM array and a JSON object as an XDM map.
Probably the main use cases I've seen for maps are
(a) to capture the result of parsing JSON input, when the input data is in JSON
(b) to construct a structure that can be serialized as JSON, when JSON output is required.
(c) to provide complex input parameters to functions (like the fn:transform() or fn:serialize() functions)
(d) to capture multiple results or compound results from functions, e.g. a function that computes both the min and max of a sequence. If maps had been available at the time, they could have been used to get the namespace context of an element much more elegantly than the in-scope-prefixes/namespace-uri-for-prefix mechanism.
(e) a map whose entries are functions can be used like an object in OO languages, to achieve polymorphism -- especially useful in XQuery which lacks XSLT's template rule despatch mechanism. The fn:random-number-generator() function design illustrates the idea.
(f) a map can act as a simple struct for compound values, e.g. complex numbers. (It could have been used for date/time/duration/QName if available, or for the error information available in a catch clause)