Brief Summary
If javascript bookmarklets are placed in the bookmark-bar it's super convenient to trigger these using shortcuts like cmd-{number}.
Details:
Bookmarks not only support http-urls, they also support other schemes like javascript (called bookmarklets). As such a bookmark is a very simple way of scripting any webpage. Some bookmarklets are so convenient that it's helpful to trigger them using a shortcut. Long ago before Tabs were even a thing, Safari had shortcuts cmd-1 through cmd-0 to quickly open a bookmark. If these bookmarks were bookmarklets, it was a super fast way of triggering a script on the current webpage.
Even to this day Safari includes a setting to not switch tabs when using cmd-{number} in order to trigger bookmarks. Thinking about it, this is the single feature that prevents me from switching browsers because other browsers dont allow me to quickly run predefined scripts on webpages.