Paul said, that to the jews(i.e. when it comes to the jews) I am as a jew, so that I may win them over.
So from his mouth, that is a reason why he did it
Corinthians 9:20 - quoted here, 9:19-22
19For though I be free from all men, yet have I made myself servant unto all, that I might gain the more.
20And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law;
21To them that are without law, as without law, (being not without law to God, but under the law to Christ,) that I might gain them that are without law.
22To the weak became I as weak, that I might gain the weak: I am made all things to all men, that I might by all means save some.