If I testify about myself, my testimony is not valid.
No-one likes someone who blows their own horn. Jesus had a 'sign' ministry. The miracles spoke for themselves. Had he just worked on the Sabbath or made what were considered blasphemous statements about himself the the Jews would have felt very safe in stoning him. But the signs were there or all to see. They would later accuse him of doing these miracles by devilish means, but while the people were behind him their hands were tied.
Jesus would go on to develop the theme that there were yet other witnesses to who he was.