Map and dictionary data types

Inside of a map, each key has an associated value and that value's type is preserved , the values stored in a map are not wrapped in variants while a dictionary wraps a variant AnyType around each value in the dictionary. Map makes it much easier to get values back out of maps by indexing, without having to worry about type casting. There are no advantages to using dictionaries; prefer using maps over dictionaries where possible.

docs.appian.com/.../fnc_system_map.html

As per your second part of the question it depends on how you are manipulating your data, some function returns map for ex a!queryRecordType() returns a map.