In British law, there is a concept known as "statutory interpretation", wherein a judge can use one of 4 methods to determine how to apply the law. One of these methods is called the "purposive approach". The purposive approach is when the judge looks into the intended purpose of the law. What did the government intend when it made the law?
In Mark 2, we see Jesus using this very method to justify healing on the Sabbath. Though the Law clearly forbids working on the Sabbath, Jesus healed on the Sabbath, and this offended the Jews. But Jesus pointed out that the Sabbath was never intended to hinder man, but to benefit him. It is a day on which man is required to take a break from his work, allowing his body to recover and prepare for the next 6 days of work. Furthermore, it allows families to spend some much needed quality time together, strengthening their familial bond. The Sabbath was never intended to stop people from responding to emergencies, or doing good to each other. Thus, one must use logic when figuring out how to apply it.
The purposive approach is typically frowned upon in law, mainly because it takes power from the legislature and places it in the hands of the judiciary. Thus, it is used sparingly. In much the same way, we should also use it sparingly, as Christians, when trying to determine how to apply the Scriptures to our lives. That being said, we do have implicit permission to interpret the Scriptures using this method. Sparingly. Cautiously. Indeed, where no purpose is explicitly given, one may argue no purpose should be sought.
It does, however, provide us with a useful way to look at the Old Testament law. We already know that the law was fulfilled at the cross. It was our guardian until Christ came, and we are no longer under that guardian (Galatians 3:24-25). This makes it easier to use this approach on the Old Testament. We can, for example, ask why the Old Testament placed such a heavy emphasis on dietary laws without being judged for breaking those dietary laws. Under the New Covenant, all foods are clean (e.g. Mark 7:19; Romans 14:1-4, 14; 1 Timothy 4:1-5). Thus, it does no harm to suggest, for example, that perhaps it is no coincidence that every animal God declared "unclean" actually has a greater health risk attached than the clean ones.
This approach is far more dangerous when applied to the New Testament. We cannot say, for example, that since the command to honor one's parents was intended so that adult children could care for their elderly parents, that therefore it is fine to dishonor our parents if they are capable of surviving on their own, or if we can find a care home to dump them in. We cannot say that abstinence was intended to prevent children being born to unprepared parents, therefore premarital sex is fine as long as you use contraception. We can't even say, as the Mormons do, that wine was permitted in the first century only because clean water was difficult to access, and therefore wine is no longer morally acceptable, and we must use water in communion.
The purposive approach is, therefore, a double edged sword. It can help us gain interesting insight into the Old Covenant, and in some cases can even improve our spiritual lives, but it can also lead to sin and lawlessness, and should therefore be used sparingly. I would even go as far as to say that when the Bible does not use it, neither should we, but when the Lord Himself uses it, there is no reason we should not also.