user1398: Links subject to link rot or the content of the link may change, therefore, all contents on SE must stand by itself even if the link is gone. However, even if the content can stand by itself, the link usually has some interesting additional content that is not related to the question/answer at hand, and sometimes, they are cited as the source of some claims or quotes.
It is inappropriate to quote everything in the post since it makes the post bloated and irrelevant, not mentioning possible copyright violation. Still, it might be interesting for future readers to check out the full content of the link or cross-check the cited source.
Some examples:

What is the relation between the female protagonists of these two manga series? (Since we are not sure whether it is true, the content might be removed on TvTropes in the future, leaving use without context).
What is the name of the manga in the bottom right of this picture? (The link to Shounen Sunday is going to rot extremely soon)

Therefore, while it is not required, it is nice to save a copy of the page on the date that the question or answer is posted to preserve the context/source of citation so that the content on SE still makes sense in the future.
Is there any convenient way to archive a copy of an external webpage?

user8486: SE developers could make use of some tools to combat link rot. These tools include amber, a server-side plug-in that gives visitors a fallback option (mirrors) if links go dead, and robustify.js, a script that works in combination with a server-side script that checks whether links still work as they are being clicked and, if not, redirects the user to a snapshot in a web archive. Of course, a snapshot has to exist prior to the redirection; there are many web archives that can take snapshots of webpages, and versioning systems can take them automatically.

A local solution might be the Mozilla Archive Format addon for Firefox. It can save and view webpages in both MHT and MAFF formats. An MAFF file is basically just MHT files compressed into one ZIP file, but opening it opens all saved webpages automatically in Firefox. One may also take screenshots and upload them to the i.stack.imgur account for better safekeeping, but the downsides of this approach should be very obvious.

user1398: Currently, I only know of archive.org Web Archive. Go to https://archive.org/web/ and paste the link in the box below to ask archive.org to save a copy of the page right now.

Image taken from this answer on webapps.SE

1398: <p>Links subject to link rot or the content of the link may change, therefore, all contents on SE must stand by itself even if the link is gone. However, even if the content can stand by itself, the link usually has some interesting additional content that is not related to the question/answer at hand, and sometimes, they are cited as the source of some claims or quotes.</p>

<p>It is inappropriate to quote everything in the post since it makes the post bloated and irrelevant, not mentioning possible copyright violation. Still, it might be interesting for future readers to check out the full content of the link or cross-check the cited source.</p>

<p>Some examples:</p>

<ul>
<li><a href="https://anime.stackexchange.com/questions/17199/what-is-the-relation-between-the-female-protagonists-of-these-two-manga-series">What is the relation between the female protagonists of these two manga series?</a> (Since we are not sure whether it is true, the content might be removed on TvTropes in the future, leaving use without context).</li>
<li><a href="https://anime.stackexchange.com/questions/17183/what-is-the-name-of-the-anime-manga-in-the-bottom-right-of-this-picture/17184#17184">What is the name of the manga in the bottom right of this picture?</a> (The link to Shounen Sunday is going to rot extremely soon)</li>
</ul>

<p>Therefore, while it is not required, it is nice to save a copy of the page on the date that the question or answer is posted to preserve the context/source of citation so that the content on SE still makes sense in the future.</p>

<p>Is there any convenient way to archive a copy of an external webpage?</p>
 8486: <p>SE developers could make use of some <a href="http://robustlinks.mementoweb.org/tools/" rel="nofollow">tools</a> to combat link rot. These tools include <a href="http://amberlink.org/" rel="nofollow">amber</a>, a server-side plug-in that gives visitors a fallback option (mirrors) if links go dead, and <a href="https://github.com/renevoorburg/robustify.js" rel="nofollow">robustify.js</a>, a script that works in combination with a server-side script that checks whether links still work as they are being clicked and, if not, redirects the user to a snapshot in a web archive. Of course, a snapshot has to exist prior to the redirection; there are many <a href="http://timetravel.mementoweb.org/about/#find" rel="nofollow">web archives</a> that can take snapshots of webpages, and versioning systems can take them <a href="http://robustlinks.mementoweb.org/spec/#data-attributes" rel="nofollow">automatically</a>.</p>

<hr>

<p>A local solution might be the <a href="https://addons.mozilla.org/en-US/firefox/addon/mozilla-archive-format/" rel="nofollow">Mozilla Archive Format</a> addon for Firefox. It can save and view webpages in both MHT and MAFF formats. An MAFF file is basically just MHT files compressed into one ZIP file, but opening it opens all saved webpages automatically in Firefox. One may also take screenshots and upload them to the i.stack.imgur account for better safekeeping, but the downsides of this approach should be very obvious.</p>
 1398: <p>Currently, I only know of <a href="https://archive.org" rel="nofollow noreferrer">archive.org</a> Web Archive. Go to <a href="https://archive.org/web/" rel="nofollow noreferrer">https://archive.org/web/</a> and paste the link in the box below to ask archive.org to save a copy of the page right now.</p>

<p><img src="https://i.stack.imgur.com/F2cv1.png" alt=""></p>

<p><sup>Image taken from <a href="https://webapps.stackexchange.com/questions/57374/get-archive-org-to-save-the-latest-copy-a-page-its-copies-of-this-page-are-ver/65760#65760">this answer</a> on webapps.SE</sup></p>
