When would you use document.write()?

In practice: never in new code. It exists for compatibility, and every use has a better replacement.

document.write('<p>Hello</p>');   // during parsing: inserts here
// after load: WIPES the entire document first

Why it is bad

• It destroys the page if called after load. document.write opens an implicit document.open() on a closed document, which clears everything — your DOM, your listeners, your state.
• It blocks the parser. The browser must stop, parse the injected string, and cannot use its speculative preload scanner past that point. Chrome outright refuses document.write of a blocking script from a third-party origin on a slow connection.
• It is a string, so it is an XSS hole the moment any user input reaches it.
• It does nothing in XHTML or in a document served as XML.

What to use instead

el.textContent = value;                      // text
el.append(node);                             // nodes
el.insertAdjacentHTML('beforeend', trusted); // trusted markup only
const t = document.querySelector('template'); // reusable fragments

To inject a script, create the element:

const s = document.createElement('script');
s.src = url; s.async = true;
document.head.append(s);

The honest answer to "when"

Two places you still see it legitimately: older third-party ad and analytics snippets that must run synchronously during parse, and a throwaway one-liner in the console or a codepen. In an interview it is fine to say you know what it does and that you would not ship it.