XQuery vs XML Query Algebra

Reading through the XQuery Semantics, I was struck by how similar XQuery is
to the XML Query Algebra -- often, the mapping is just a trivial change of
syntax. So that got me wondering: if we have the Algebra, why do we need
XQuery?  Or, to put it another way, does XQuery fulfill the XML Query
Requirements any better than the Algebra would?

Not contentious, just curious,
-Michael Dyck