I don't think this is a well-formed question, because DRM is generally code
implementing 2). That is, things DRM prevents you from doing are pretty much by definition things that are not within the guidelines of publishers.
But to treat the question charitably, here's one use case: I want to view the media on whatever devices I happen to own. That is, I want to e.g. start reading a e-book on my PC, continue on my Kindle, and then finish it on my smartwatch. Or my laser projector. Or my fishbowl. Also maybe I think the book's font is simply ugly, and I want to change it. Oh, and my Internet connection is pretty shitty, because I live in a developing nation and/or US rural area, so I'd love to be able to do all of that off-line at times[0].
That was a basic use case. To step it up, imagine I'm doing research. I want to do notes and cross-referencing, and I want to efficiently move around the text. Regular DRM-ed formats tend to make that nigh-impossible[1].
> How would that example translate to a non-DRM version while making sure that the library isn't distributing an unbounded number of licenses to customers?
It wouldn't, because DRM is by definition designed to limit you to only a very narrow way of consuming a piece of digital media. Any kind of additional flexibility will make it possible to "distribute an unbound number of licenses" for free.
Here's the fundamental conflict - as a consumer, I want to buy/rent a piece of content, as narrowly constrained as possible. In particular, I want the publisher to stop messing with the way I use it and the format I chose to consume. OTOH, publishers have exactly opposite interests - they want to have as much control over my consumption as possible, whether to make more money, to protect against copying, or both.
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[0] - Related, it's kind of ridiculous to have to stream a book from the Internet all the time. DRM does encourage some pretty shitty engineering decisions.
[1] - Consider just how much economic damage is being done to society by using DRM to make media usable for lowest common denominator of consumers, while making it much more difficult to use in a specialized way for interesting or important work. It's a huge waste, to take a general-purpose computer and use its power to restrict its own functions.