Buttons can have a value like input elements

The <button> element can have a value like <input> and this value can also be passed to the server when the form is submitted:

<form action="#" method="POST">
<input type="text" name="name" value="Peter" />
<button type="submit" name="_action" value="add">Add</button>
<button type="submit" name="_action" value="delete">Delete</button>
</form>

The form can be submitted with this data without the need of JavaScript. On the server, you can check the value of _action to decide what to do next based on which button the user clicked to submit the form.

If you are submitting the form programmatically using JavaScript, FormData needs to know which button was used to submit the form:

document.querySelector("form").addEventListener("submit", (event) => {
event.preventDefault();

const formData = new FormData(event.target);

// The FormData does not know how the form was submitted.
// There could be more than one submit button on the form and
// we want to include the one that was used to submit the form.
// We append the button (also known as the submitter) to FormData.
// See more: https://developer.mozilla.org/en-US/docs/Web/API/SubmitEvent/submitter
if (event.submitter) {
formData.append(event.submitter.name, event.submitter.value);
}

const data = Object.fromEntries(formData.entries());
console.log(data); // {name: "Peter", _action: "add"}

// When ready, submit the form programmatically
});

Remix does a great job at using this technique - they demonstrate it in their video Remix Single: Multiple Forms and Single Button Mutations. The Remix implementation can be found here.