Good question. Let’s try this.
We are talking about believing in a God, even the only God. God made everything. So, it is not a matter of what make sense to us, so we believe. But in this case, it is what He tells us who He is, and we either accept it or we don’t.
Otherwise, he is not God, since we don’t care so much about what He says, but what we think make sense.
Okay?
Now, He claims that He is One and Only. Yes.
But then when Jesus came, He affirmed that God is One, but followed by He is God’s Son, of the same substance, declaring God and Himself are One and the same reality (see John 10:30).
Obviously Jesus and God the Father are distinct, yet they are one. This is coming from GOd Himself. We have at least a duality.
But then in John 13-17, Jesus repeatedly mentioned the Holy Spirit as coming from both God and Himself, and will do what God does. Now GOd is telling us He is Three and yet One.
That’s why we have the Trinity, because God told us that is Who He is.
But if you mean what it means to us that God is not unitarian, but trinitarian…
One way to think about this is that the Bible says God is love. One person cannot love. It takes at least two to love. If God is unitarian, then until He created the universe and the angels and us, He was really incomplete, since He lacked us to become who He is, love. His creation then became a necessity and in a way He needs us to fully be. His love and creation, therefore, would not be entirely unconditional and gratuitous. It would not be an entirely free act.
But God is not unitarian. He is trinitarian. Within Himself are three Persons perfectly in giving and receiving love. He in Himself is completely perfect and fulfilled. He has no need to create to be love. And so the fact that He did choose to create reveals to us that this creation is absolutely and completely free, over the top gratuitous and risk-taking in pouring Himself out of His superabundance goodness. And that, makes love truly love: it is totally unnecessary, but a pure act of choice to give us everything, just because He chooses us.