Bereshit 12 (verse 2-3) reads: "I will make of thee a great nation, I will bless thee and make thy name great; be thou a blessing. I will bless them that bless thee, and him that curseth thee will I curse, and in thee shall all the families of the earth be blessed."

These words are repeated in other verses as well. Why is it that HaShem (Who first declareren to Avram to leave his country, kindred, father's house, and go to the land which HaShem will show him - to go to) declares to Avraham: 'to become a nation' and 'in thee shall all be blessed'?

Why was it neccessary to become a nation and a blessing? What's the impotance behind it?