It provides ways to interact with ebooks protected with DRM systems. It's basically the only mainstream tool in that space; integrates easily with readers and collection managers, it's very battle-tested, has been around for a long time.
noDRM published its code on GitHub. A DRM developer now claims it provides illegal tools, so GitHub disabled the repo.
The community-developed LCP software is used by public libraries (mostly in Europe) to lend ebooks to patrons fora limited period of time, say one month.
What NoDRM allows is for that lending period to be broken, so that patrons (readers) can read the ebook they have borrowed forma public library indefinitely.
The practical consequence is that copyright owners (authors, publishers., etc) would in most cases cease to make ebooks available for lending.
In some cases Readium LCP is also used by ebook vendors (retailers), mostly small European ones, as an alternative to Adobe's RMSDK, but the LCP platform is constructed such that if you buy the ebook from one vendor that you can read it on the app of any other vendor using Readium LCP, even if their authentication server is different (Adobe always requires authentication through a central Adobe server, meaning the end user needs an account both with Adobe AND the vendor/library, something not required with Readium LCP). IT is the most interpoerable system available in the publishing ecosystem
noDRM published its code on GitHub. A DRM developer now claims it provides illegal tools, so GitHub disabled the repo.