Why Do I Need a Denomination?
You don't have to "have a denomination", but fellowship with like-minded believers is important for one's spiritual walk. The only way you're going to find people who believe what you believe to be true is to assign some understandable name to your common beliefs.
While many matters of disagreement are matters of conscience which we should not let divide us, other differences are matters of doctrine, which are important to agree on. Few would want to go to a church where or where their beliefs are preached against. Some denominations which called themselves Christians are not even viewed as Christian by others. Ensuring a common definition of what it means to be a believer is obviously important.
2 Corinthians 6:14 Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?
If God is the most important thing in your life and has made your life better, surely you would want others to know Him as you do, not a false version of Him. As God is eternal and unchanging, there are objective truths about Him and objective falsehoods. A disciple will want to share these truths with others, not participate in the propagation of something they believe to be false. In fact, the Bible specifically tells us there will be false teachers in churches and that we must avoid those who teach false doctrine.
2 Peter 2:1 But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction.
Romans 16:17 Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned; and avoid them.
How can two people teach together in a church when they teach different messages about core doctrines? Even as a practical matter, you have to agree on what to do in order to work together.
Amos 3:3 Can two walk together, unless they are agreed?
Again, you will never agree on every little thing, but it's important to find a group of people you can agree on the important things with so you can help each other accordingly. The benefit of "having a denomination" is that it makes it easier for you to have a local church, the importance of which is underscored throughout the New Testament. Without other believers, you can have no accountability and likely little edification.
Proverbs 27:17 Iron sharpeneth iron; so a man sharpeneth the countenance of his friend.
Ecclesiastes 4:9-10 Two are better than one; because they have a good reward for their labour. For if they fall, the one will lift up his fellow: but woe to him that is alone when he falleth; for he hath not another to help him up.
1 Thessalonians 5:11 Wherefore comfort yourselves together, and edify one another, even as also ye do.
Hebrews 10:24–25 And let us consider one another to provoke unto love and to good works: Not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching.
More importantly than just receiving edification and encouragement, without being around other Christians, you can't build up others yourself. The Christian life is not about living for yourself but about living for others. To help other believers, you have to be in a community of other believers.