why using blur event instead of submit for form data submission? – Shopify

I was reading klaviyo’s integration guide. The javascript it is using to submit form data is:

<script type="text/javascript">
    $("input[name='contact[email]']").on('blur', function(e) {
        e.preventDefault();
        var email = $(this).val();
        var settings = {
            "async": true,
            "crossDomain": true,
            "url": "https://manage.kmail-lists.com/subscriptions/external/subscribe",
            "method": "POST",
            "headers": {
                "content-type": "application/x-www-form-urlencoded",
                "cache-control": "no-cache"
            },
            "data": {
                "g": "LIST_ID",
                "$fields": "$source",
                "email": email,
                "$source": "Shopify form"
            }
        };
        $.ajax(settings).done(function (response) {
            console.log(response);
        });
    });
</script>

I am wondering why it is using the “blur” event instead of user’s clicking on “submit” button to trigger the ajax function. I think with this code, if user opt out of the form’s text-box prematurely without clicking “sign up” or “submit”, the data will be submitted.