Not sure, but is it not that bookmarklets need to be clicked by user on every page visit, whereas Extension can auto process every page visit without user intervention?

Good point. Although a bookmarklet can also ignore the current site and just open a new tab, it can't embed custom JavaScript into that new tab.

But at least it can open a new tab with a full-page iframe, which was just enough for me to implement my autoreload bookmarklet. [1]

Apart from that, I believe there are meta-extensions which provide a generic way to inject JavaScript into every page. This is just one extension, but manages multiple bookmarklets (or whatever they call these snippets).

[1] https://vog.github.io/autoreload/

bookmarklet to make all text black, no exception.

    javascript:(function(){var%20i,x;for(i=0;=document.getElementsByTagName("*")i];++i)x.style.color="#000000";})();

No need for a bookmarklet. It can be easily done with CSS.

What's the difference in this specific case? How do I automatically apply a custom CSS onto an external website? Is this somehow simpler than injecting JS into an external website?

tampermonkey's good