Why should we concern ourselves with Old Testament practices of the Jews? Because they are a shadow or a pattern of the reality that was to come. In many cases, you can not accurately understand the New Testament without understanding the Old. Many false doctrines today come from a refusal to completely understand the whole book. I do not know who said it, but the phrase "The New is in the Old Concealed, the Old is in the New Revealed" states the case most eloquently. The Messiah said that He did not come to destroy the law and the prophets but to fulfill. It is therefore our duty to understand how the two testaments relate.