Bitcoin's job is to be money. A fixed supply, secure store of value, simple by design. People hold it and send it. That's basically it. The simplicity is a feature, not a limitation. It's been online without a single hack since 2009.
Ethereum's job is to be a computer. Anyone in the world can run programs on it called smart contracts. That's why almost everything in crypto, from stablecoins to NFTs to most apps, runs on Ethereum or networks built on top of it. ETH is the fuel that pays to run those programs.
Quick mental model: Bitcoin is digital gold. Ethereum is digital infrastructure. They're not really competitors, even though their prices often move together. Most people who hold one end up holding both.