Does Preordering an eBook Make Any Sense? No, but we do it anyway. 

     Preordering is what a reader does to ensure a copy of a book that is expected to quickly be in short supply after publication, a way to make sure you get yours when everybody wants one. This is a consideration in offset printing, where a specific number of copies are printed to satisfy the expected number of purchases on publication day. If the author is popular and buyers consider it possible that the currently printed copies might sell out fast on publication day, they might preorder it to have the opportunity to get first dibs on publication day before vendors run out of the initial run.
     This does not apply to eBooks. They are, by definition, never out of stock. Publishers never run out of them because they don't exist until they are are ordered. Order an eBook, and it's on its way to you instantly as a device file with no depletion of inventory for the publisher. The availability of the book is unlimited on the day of publication, and it will always be unlimited.
     As a practical matter, you, the customer, you have no incentive whatsoever to preorder an eBook; it won't be available until the official publication day whether you order it then or preorder it a month earlier. You won't get it any sooner, and you won't get it any cheaper. You're at no risk of not being able to get your copy on pub day, just like everybody else. The publisher will never run out of stock.
     The only benefit that might accrue from this type of preorder is to the publisher and the author by generating some buzz around perceived demand for the title, by conveying the notion that it might become more difficult to get as publication day approaches. It won't be. You can order it anytime and get it just as quickly as at any other time. If you want to support a particular eBook author or publisher, preordering might be a good way to do that because preorders count as first-week sales, so it might cause a boost in initial reported sales, albeit perhaps not a precisely accurate one, time-wise.
     Preordering an eBook won't help you, but it won't hurt you, either. It's of no use to you as a reader, but it might make some difference in the author's sales ranking if that is of any concern to you. If you want to read a little of the book at Amazon's bookstore, you cannot do that, because the printer source file of a preordered eBook typically hasn't been delivered to the publisher yet, so you won't see the usual “Read a Sample” link below the cover picture on the book's Amazon page until it's officially on sale.