Why the law to begin with?

"Clearly, God’s promise to give the whole earth to Abraham and his descendants was based not on his obedience to God’s law, but on a right relationship with God that comes by faith. If God’s promise is only for those who obey the law, then faith is not necessary and the promise is pointless. For the law always brings punishment on those who try to obey it. (The only way to avoid breaking the law is to have no law to break!) So the promise is received by faith. It is given as a free gift. And we are all certain to receive it, whether or not we live according to the law of Moses, if we have faith like Abraham’s. For Abraham is the father of all who believe. That is what the Scriptures mean when God told him, “I have made you the father of many nations.” This happened because Abraham believed in the God who brings the dead back to life and who creates new things out of nothing." Romans 4:13-17

The law was given to Moses not so that the people would work very hard to follow it and do their very best to up hold it, even though that is exactly what happened. They quickly found out that the law was un-keepable. Impossible even! The Pharisees tried very hard and accomplished in transforming the law into a religion that was impossible for the Jews to live by. Only sinning that much more the harder they tried.

So why did God give them/us the law to begin with if it is impossible to up hold?

We find the answer in Matthew 5:17-18;
“Don’t misunderstand why I (Jesus) have come. I did not come to abolish the law of Moses or the writings of the prophets. No, I came to accomplish their purpose.  I tell you the truth, until heaven and earth disappear, not even the smallest detail of God’s law will disappear until its purpose is achieved."
Jesus came not to take away the law but to fulfill it. He came because the law was created to be impossible to prove our need for a savior. He came because He is that savior! See God knew that we would keep on sinning and keep on drifting farther and farther from Him. He is holy and perfect therefore He could not look upon us because of our sins. Jesus came to pay for our sins  as the last blood sacrifice so that we can have a personable relationship with God. Jesus came to make salvation reachable and easier.
Look at verse 18 again. God clearly instructs us to not take the law lightly. He still expects us to do our very best to up hold it, and a lack of doing so is a sin. One day we will face God for it. But the law is not the road to salvation. Faith in Jesus Christ is the only way to heaven.
"Jesus told him, “I am the way, the truth, and the life. No one can come to the Father except through me." John 14:6
The law is to guide us to be more like  Jesus. Jesus is to make us holy and pure to God.