These verses do not appear in Matthew"s parallel account. They explain Pharisaic tradition for those unfamiliar with it such as Mark"s original Gentile readers. In Jesus" day the Jews communicated the traditions of the elders orally from generation to generation. About A.D200 the rabbis completed compiling these into the Mishnah, which became the basis for the Talmud (ca. A.D425). The Pharisees customarily washed themselves after visiting the marketplace to rid themselves of the defilement that contact with Gentiles produced. Most Jews regarded breaking these traditions as sin. [source][source][source]
"Indeed, a Rabbi who had held this command in contempt was actually buried in excommunication." [1][source]