Let’s unpack first what it means for God to be metaphysically simple. In classical theism, divine simplicity means that God’s essence (the nature of God) is identical to his existence. The way I understand this is that you can’t imagine God having different properties and attributes than what he already has, otherwise he wouldn’t be God, a but a different type of being entirely. To be God is to exist as God.
Now, if God is not metaphysically simple, then he would be metaphysically composite, having his existence differentiated from his essence. Thus, it is possible to imagine multiple instances of God existing at the same time with slightly different accidental properties. This would downgrade the classical God into merely one of a pantheon of gods. To take an example, Zeus and Poseidon are both gods in the Greek conception, even though they have slightly different properties - for example, Zeus uses thunderbolts, Poseidon uses his trident. These are accidental (not essential) properties of being divine in the Greek system. You could imagine Zeus using a trident and he would still be Zeus.
In contrast, the classical God doesn’t have any accidental properties. He just is God, and if you change anything about him then he wouldn’t be the same being. Divine simplicity also entails divine unity: there is no accidental property you can change about God to differentiate him from another instance of him, so God is necessarily one.
Why would we care if God is downgraded to merely one of the multitude of pagan gods? Because if some of his properties could change while still being God, then he would no longer be a necessary being, but merely a contingent one. He would need a cause of his existence, just like everything else. He would not be able to be the Prime Mover (i.e. creator and sustainer) of everything else in existence. He would no longer truly be God, but merely a super-powerful alien being.
 This is also why the popular internet atheist argument that “I don’t believe in Zeus or Odin, why should I believe in God?” falls flat for believers of classical theism, for the God of classical theism is a very different being than Zeus or Odin.
 And this makes sense in real life: Thor, who is not the classical God, but a pagan god, has become a Marvel superhero, and it doesn’t feel very jarring. He has great powers but is still limited by the laws of the universe, and dependent on tools and weapons to express his power.