I see two purposes at least. Isa. 66 finds us at the New City in Worship of God, and the City finds NO NEED OF THE SUN THERE!;) Even the whole earth will never again be in total darkness. But the New Heavens & Earth will be as at the beginning with 'our day' divisions. (Eccl. 3:15) Before the flood the earth had a watering system different than that of raining even. But here is the first purpose of why we were born. It was Adams FIRST day of Loving Worship, yet on the seventh day of Earth's creation. God has stated that for a fact. Gen. 2:2-3!
Isa. 66
[22] For as the new heavens and the new earth, which I will make, shall remain before me, saith the LORD, so shall your seed and your name remain.
[23] And it shall come to pass, that from one new moon to another, and from one sabbath to another, shall all flesh come to worship before me, saith the LORD.
OK: After WORSHIP?? 'i' see Isa. 65:21-25 telling me of our country homes + farming? Vineyards at least, and LOVING TO WORK! And CALLING on GOD finds Him, ANSWERING US BEFORE WE CALL!
SO: That is for then, and I see 'us' to finally be seen there, we will be doing the same type of things today. Of course [WE] now have the Matt. 28:19 +,+,+ verse *20* to do because we Love our Master & the ones that He died for.
And let me just add, that our motives might be misunderstood while on this present earth by even your question including Christians, but Christ knows what ones True Motive is, huh! Jude 1:22-23.
--Elijah