Usage scenarios of 'treat as'

Hi,

I'm curious on usage scenarios for the 'treat as' expression, in particular
for when XSL-T 2.0 is the host language(if that matters).

In XPath 2.0 is verification of an operand's type(the function conversion
rules) done at runtime(implementation dependent if guaranteed runtime type
errors are detected statically), unless the implementation implements
"pessimistic" static type checking. (Right?)

In what case is the 'treat as' expression useful, or required, when the
implementation does not do pessimistic, static type checking? (that is, usage
scenarios which applies for all implementations regardless of what optional
features that are implemented.)