One use case for session-based namespace remapping occurs in the context of an XPath or SQL query (see 6.6 Searching Repository Content and 8.58.5 Searching Repository Content with SQL).
Queries often include literal names that have namespace prefixes. When attempting to use a stored query (or one obtained from some external source) whose prefixes do not match those currently used in the repository, dynamic remapping of namespaces in the session allows the temporary session mapping to be adapted to whatever prefixes are used in the query statement.