title: What is Vue.extend for?
body: <p>This is a simple enough one, but I'm not getting the answer I need from the documentation. I'm in the process of learning Vue.js, and I don't really understand where Vue.extend fits in. I get Vue.component, but I don't see what <a href="https://v2.vuejs.org/v2/api/#Vue-extend" rel="nofollow noreferrer">Vue.extend</a> does which <a href="https://v2.vuejs.org/v2/api/#Vue-component" rel="nofollow noreferrer">Vue.component</a> doesn't. Is it just a legacy feature from 1.0? If not, where is it useful in Vue 2.0?</p>