As with much of the Bible. What we have to do is to take it in relationship to other parts of the Bible. In the following excerpt from Darby's commentary on the full Bible, we can learn more about what St. Paul was talking about.
Most assuredly God did have a reason for creation. In trying to discern what God's reasoning was for the creation and we have to take the entire Bible, and use it to discern exactly what the whole Bible is trying to tell us.
In the book of Ephesians, Paul is defending his ministry to the Gentile nations.
Paul was imprisoned in Rome waiting to be tried before Caesar, and the letter to the Ephesians (a Gentile Church) was intended to bolster their faith and assert their right to share in the Kingdom of God.
Ephesians 3:10 through12 To the intent that now unto the principalities and powers in heavenly places might be known by the church the manifold wisdom of God, According to the eternal purpose which he purposed in Christ Jesus our Lord: In whom we have boldness and access with confidence by the faith of him.
All Scripture is quoted from the King James translation.
The design of God in giving Paul grace to proclaim to the Gentiles the mystery of
salvation heretofore hidden.
now first: opposed to “hidden from the beginning of the world”
Ephesians 3:5 Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;).
unto the principalities and (Greek adds “the”) powers unto the various orders of good
angels primarily, as these dwell “in the heavenly places” in the highest sense; “known” to their adoring joy.
1st Timothy 3:16; And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.
1st Peter 1:12 Unto whom it was revealed, that not unto themselves, but unto us they did minister the things, which are now reported unto you by them that have preached the gospel unto you with the Holy Spirit sent down from heaven; which things the angels desire to look into.
Secondarily, God’s wisdom in redemption is made known to evil angels, who dwell
“in heavenly places” in a lower sense, namely, the air
Ephesians 2:2 Wherein in time past all of you walked according to the course of this world, according to the prince of the power of the air, the spirit that now works in the children of disobedience:
Ephesians 6:12 For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places.
“known” to their dismay
1Corinthians 15:24 Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power.
Colossians 2:15 And having spoiled principalities and powers, he made a shew of them openly, triumphing over them in it.
might be known — Translate, “may be known.”
by the church — “by means of,” or “through the Church,” which is the “theater”
for the display of God’s manifold wisdom:
Luke 15:10 Likewise, I say unto you, there is joy in the presence of the angels of God over one sinner that repenteth.
1st Coriinthians 4:9 For I think that God hath set forth us the apostles last, as it were appointed to death: for we are made a spectacle unto the world, and to angels, and to men.
“a spectacle (Greek, ‘theater’) to angels.”
Hence, angels are but our “fellow servants”
Revelation 19:10 And I fell at his feet to worship him. And he said unto me, See thou do it not: I am thy fellowservant, and of thy brethren that have the testimony of Jesus: worship God: for the testimony of Jesus is the spirit of prophecy.
manifold wisdom — though essentially one, as Christ is one, yet varying the economy
in respect to places, times, and persons
Isaiah 55:8 and 9For my thoughts are not your thoughts, neither are your ways my ways, saith the LORD. For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts.
Hebrews 1:1 God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets,
1st Peter 4:10 As every man hath received the gift, even so minister the same one to another, as good stewards of the manifold grace of God.
“stewards of the manifold grace of God.”
Man cannot understand aright its single acts till he can survey them as a connected whole
1st Corinthians 13:12 For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.
The call of the Church is no haphazard remedy, or afterthought, but part of the eternal
scheme, which, amidst manifold varieties of dispensation, is one in its end.
So in answer to your question Ephesians 3:10 is only a small window into the purposes of God in his creation. There are mysteries hidden in the Universe, which we are just now beginning to discover. We do not as yet know if God created other life on other Planets or not, and what impact that has on our destinies. Until we know all of God's mysteries we will never really know God or his purposes.