My /etc/apt/apt.conf file contains only:
APT::Default-Release stable;
Which double quotes are you talking of?
As for the purpose, it's about updating libc6-dev. There's a conflict (the "memcpy@GLIBC_2.14" thing) with SFML and the libc6 provided with Debian 7. Doing this testing hack allows getting a more recent libc6-dev package that get things working fine with SFML (at least, that's my understanding).