That thing has its own utility. But sometimes there is this one note inside the notebook which need to be in top (say pinned) all the time. For a example, Notebook 'Economics 101' has many notes in it. One particular note is about 'Syllabus and Timeline' which need to be in top irrespective of how the other notes are sorted (by name or date). You want it in top and also want other notes to be sorted on the latest update basis. In this case, pinning the note comes handy.
Many messaging Apps has this feature. Like in Telegram, you have an option of folders for different chats (Evernote: Notebook for different notes), you can pin particular chat(s) in particular folder (Evernote: pin particular Note(s) in particular Notebook if feature implemented).