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.

So what's preventing those people from using bookmarks as "check this out later" tool? The personal preference of using an app that reinvented those same bookmarks? Just create a "read-it-later" later directory and boom, you're good to go.

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.

Yeah, because these are features typically provided by your browser. Hence, browser bookmarks. It's not a unique feature to read-it-later apps in any way.