Many times I a faces troubles with understanding why a function is not working without putting return keyword or curly braces

for example here I have only one object after the arrow so what is the need for return and curly braces here?

function handleChange(event) {
    setFormData((preValue) => {
        return {
            ...preValue,
            [event.target.name]: event.target.value,
        };
    });
}