Many legal systems protect the rights of legal owners of copies (of copyrighted works) to backup these copies.
Licensed use of a DVD, book or other media may vary not just by legal jurisdiction but also by the specific license granted with that specific DVD/book/media. In addition, some licenses restrict the format of backup copy allowed, for example they may allow a book to be photocopied in hardcopy format but not scanned, or they may allow a CD to be copied to another CD but not extracted to MP3 files or other electronic music formats or used from mobile devices other than portable CD-players.
The original copy must be kept by the rightsholder (i.e. you can't buy a DVD, copy it while you own it, then sell the original while retaining the copy), and in order to avoid any doubt about legality of the copy, it should be marked as a backup copy including details of the rightsholder (name, purchase date etc) written on it (i.e. it should not be printed to appear like a counterfeit).
If I own a DVD or a book, I can* rip that DVD and scan that book to a PDF. But what if you where not an expert on video codecs, or did not want to buy an expensive scanner and a professional image to text software to do a scan of the book justice? You could theoretically pay someone else to copy your book or DVD.
Generally speaking with media such as film DVDs or music CDs it should be cheaper to buy a second copy than to pay someone for their time/effort to go through the hassle of making a usable (similar quality) copy. Depending on the license you may not have the rights to pass the originals or any copies onto any third party for any purpose.
But considering that your copy is identicle to millions of other copies of the original copyrighted work, what if technically your backup was a copy of one of these identicle other copies? If 100 people sent in a book to a company to copy it to a pdf for them, could that company just copy one of the books and send everyone the file? Or would the law require them to individually copy each book, creating identicle pdfs every time, and send the specific pdf to the corresponding customer?
How would the customer be able to proove to this company that they legally own a copy themselves without sending it? For the company to store a copy legally after returning the original, it would surely also need to purchase an original, otherwise they are just storing up copyright material without having purchased any licenses in the material nor obtained the permission of the copyright owners.
Expanding this, if I own a book or dvd, can I just download a backup someone else made?
The person who made the first backup has not been granted the rights to publish or distribute the copyright material by the copyright owners so they would likely be breaking the law by making it available to you, and this would likely be considered an illegal copy even if you also own an original because of how it was obtained (i.e. you didnt not exercise a right to take a backup copy from the original, you transferred an illegal copy over the Internet).
*Some of this can be a legal grey area, but that is outside the scope of this question.
Yes, other issues might include bypassing or circumventing copyright protection technologies, computer misuse laws, counterfeit laws etc.
