If you have links to another origin, you should use rel="noopener", especially if they open in a new tab/window.
Without this, the new page can access your window object via window.opener. Thankfully the origin security model of the web prevents it reading your page, but no-thankfully some legacy APIs mean it can navigate your page to a different URL using window.opener.location = newURL.
However, due to the synchronous cross-window access the DOM gives us via window.opener, windows launched via target="_blank" end up in the same process & thread. The same is true for iframes and windows opened via window.open.
rel="noopener" prevents window.opener, so there's no cross-window access. Chromium browsers optimise for this and open the new page in its own process.