I think most people these days don't use browser bookmarks as a "check this out later" tool, and instead as more of a "I frequently need to access this page" function. For me, I only bookmark a page if it's something I frequently access; things like my email, Lemmy, some work apps, etc. In my use-case, bookmarks are a more "permanent" installation to my browser.
Also, "read later" apps generally strip the web page formatting and advertisements, and usually have an offline function of some sort; both of which you typically can't do with bookmarks. These are especially useful for those who like to read on their commute.