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get-environment-variable is expected to return a “Scheme
string”.
Unfortunately, many current platforms, including POSIX–like ones, do
not require environment variables to be interpretable as sequences of
characters. In particular, environment variables can be used to name
files, and filenames on the system can amount to NULL–terminated
byte vectors, which, if the Scheme program were to collect uninterpreted
and pass to, say, the open call, would work just fine, but which might
not represent a string in the particular encoding the program expects.
While in principle it may be desirable to provide a mechanism for accessing environment variables raw, this SRFI specifies a “string” return type because that best represents the consensus of existing implementations, and captures the semantically desirable behavior in the common case that the byte sequence is interpretable as a string.