The spread syntax works with more than just arrays. From MDN's spread syntax documentation: Spread syntax can be used when all elements from an object or array need to be included in a new array or object, or should be applied one-by-one in a function call's arguments list

In the case of your code, useFormInput returns an object with the keys value and onChange. Your code takes the resulting object and spreads it into an input element. Input elements can be provided an onChange and value (among other things), and the spread syntax is used here todo exactly that. The object property name is used when including the elements (properties) from the object in the input element, meaning that inputProps.onChange will be assigned to the input's onChange and inputProps.value will be assigned to the input's value.

// Typescript definitions to simplify
// const firstNameProps = { value: string, onChange: (e) => void}
// These statements do the same thing
<input {...firstNameProps} />
<input value={firstNameProps.value} onChange={firstNameProps.onChange} />

Removing the spread syntax will try to feed your object directly to the input element, which doesn't do it much good and will cause it to throw errors.