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.

Спасибо, joba.

Пожалуйста.