В общем, в irc объяснили что пользуются этим в основном для тестов и для дебага.
http://srfi.schemers.org/srfi-6/srfi-6.html
Rationale
Scheme's procedures for performing input and output from and to ports
become more useful when extended by string ports. The interface
described here has been in use since 1986 or before, and is currently
supported by several of the major implementations.