The interfaces file is retained for compatibility with Sybase environments. It is recommended that new users use the freetds.conf format instead.
Under Sybase OpenClient there is a file called interfaces that defines servers available to the software. FreeTDS inherited this file structure with minor alterations. The interfaces remains supported for backward compatibility, and for those running in a mixed FreeTDS/Sybase environment.
The interfaces is not read by FreeTDS unless it does not find freetds.conf. Note also that make install will install a skeleton freetds.conf, which you'll have to remove if you want to use interfaces instead.