Question:
Why does God need us and what was His ultimate reason for creating us?

Answer:
The answer to your question can only be found in the Word of God. Below are a few Scriptures for your consideration before the Lord. They tell us that all things were created:

(1) For His pleasure: “Thou art worthy, O Lord, to receive glory and honor and power: for THOU HAST CREATED ALL THINGS, and FOR THY PLEASURE they are and were created” (Revelation 4:11).
(2) For Himself: “The LORD hath made ALL THINGS FOR HIMSELF: yea, even the wicked for the day of evil” (Proverbs 16:4).
(3) Every believer is “created in Christ Jesus unto good works” (Ephesians 2:8-10).
(4) God formed His people to praise Him: “This people have I formed for Myself; they shall show forth My praise” (Isaiah 43:21).
(5) God created His people for His glory: “Every one that is called by My name…I have created him for My glory, I have formed him; yea, I have made him” (Isaiah 43:7).

(“Created,” “formed,” and “made” in the verses above are three different Hebrew words, which tell of God’s working in His people so they can show forth His “glory.”)
(6) God desires “those that have made a covenant with Me by sacrifice” to gather “unto” Himself (Psalm 50:5).
All angels (except those who rebelled against Him) praise Him and shall praise Him for all eternity (Revelation 5:11, 12), but only humans who have accepted Christ’s sacrifice for sins, praise Him willingly because they have “made a covenant with” Him “by sacrifice.” Jesus told the woman at the well, “The Father seeketh SUCH to worship Him” (John 4:23).
God does not NEED anything from us. He doesn’t need wisdom, knowledge, counsel, or anything else, because “ALL THINGS” are “of…through…and to Him” as Paul exclaims in Romans 11:33-36: “O the depth of the riches both of the WISDOM and KNOWLEDGE of God! how unsearchable are His JUDGMENTS, and His ways past finding out! For who hath known the mind of the Lord? or who hath been His COUNSELLOR? Or who hath first given to Him, and it shall be recompensed unto him again? For of Him, and through Him, and to Him, are all things: to whom be glory forever. Amen.”
But as we realize that He created, possesses, and sustains us, and everything else, we can’t help but praise and worship Him, as Paul did in the verses above.
I don’t know of any Scripture that says that God NEEDS anything, but we are told that He desires and seeks worshippers. As Jesus told the repentant sinner at the well: “The hour cometh, and now is, when the true worshippers shall worship the Father in spirit and in truth: for the Father seeketh such to worship Him” (John 4:23).
Worship is the overflow of adoration to God from a heart that is taken up with Christ. Worship is to “joy in God” (Romans 5:11). PRAISE is “the fruit OF OUR LIPS” (Hebrews 13:15), but WORSHIP is to make “melody IN YOUR HEART to the Lord” (Ephesians 5:19).
I don’t know of any Scripture that says that God’s “ultimate reason for creating us” was that we would worship Him, but we are told that this was one of the reasons He created us (Isaiah 43:6, 7). It also seems, from Proverbs 8:30, 31, that He created us to have fellowship with Him: The Lord says: “I was set up from everlasting, from the beginning, or ever the earth was. . . . Then I [God the Son] was by Him [God the Father], as one brought up with Him: and I was daily His delight, rejoicing always before Him; Rejoicing in the habitable part of His earth; and My delights were with the sons of men” (Proverbs 8:23, 30-31). He showed His desire for fellowship with man from the days of Adam and all down through man’s history, and He pleads for fellowship with His people in the last church period—which we are living in today. “Behold, I stand at the door, and knock: if any man hear My voice, and open the door, I will come in to him, and will sup with him, and he with Me” (Revelation 3:20).
Proverbs 8:30 tells of the happy relationship and fellowship the Father and the Son have always had: “Then I was by Him, as one brought up with Him: and I was daily His delight, rejoicing always before Him.” Proverbs 8:31 goes on to tell us: “Rejoicing in the habitable part of His earth; and My delights were with the sons of men.” So it seems clear that He also created us in order to have communion and fellowship with us. This is a privilege we greatly benefit from as well.
Our finite minds are not able to fully understand God and His purposes and reasons for doing things. We only can know what is revealed to us in His Word. God says: “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” (see Isaiah 55:7-11).Though we know from God’s Word that He made us to worship and have fellowship with Him, there are some things that we may never know the answer to (at least not this side of the glory). Your question may fall into this category. “The secret things belong unto the LORD our God: but those things which are revealed belong unto us and to our children forever, that we may do all the words of this law” (Deuteronomy 29:29).