Some people like to leave their comments on your post with website url, I think most probably they want to get link juice from your comments. If you want to remove url field from WordPress comment form, perhaps you can paste the below code to your functions.php file. Once it is saved, the url field will be automatically removed from your comment form.

function remove_comment_fields($fields) {
    unset($fields['url']);
    return $fields;
}
add_filter('comment_form_default_fields','remove_comment_fields');

Thanks for WP Tuts for the great code!