For instance, an achievement that has about the same unlock rate as the "complete stage 1" achievement is probably something you stumble upon randomly in the very beginning of the game, if you don't have it, you probably missed something. If an achievement has a significantly lower unlock rate than "complete the game", it is probably end game content and may require dedication.

Achievement can have more than one purpose. They can be used as progression markers (the "tutorial complete" kind). As hints about some secondary content. As a reward for curiosity or particularly difficult tasks. As an indication that you have finished something and there is no more of it. To taunt the player. As objectives that don't fit in the story. To acknowledge that a particular action is significant. Etc...

For example "Use the Batarang for the first time" tells you that you will use it again. It hints that from now on, the Batarang will be an important part of gameplay, more important than the other items you didn't get an achievement for. On the achievement list, it is a progression marker. If you see it on an achievement list, it tells that the players is, say, at least 10% into the game, and puts other achievements in perspective.