Legal deposit in English law goes back to 1662, and the modern arrangement runs on the Legal Deposit Libraries Act of 2003. Six institutions are entitled under it to material published in the United Kingdom and Ireland: the British Library, the Bodleian, the national libraries of Scotland and of Wales, Trinity College Dublin and Cambridge University Library. Since April 2013 the entitlement covers digital publication too, so websites, blogs, electronic journals and ebooks now arrive alongside the printed sheets. The purpose the library states for all of it is that the nation's published output should be collected systematically and preserved for the use of future generations.
Cambridge is unusual among the six in how it treats what it receives. A large proportion of the material stays on open access shelves rather than in closed stores, and some categories of reader may borrow from the collections, which almost no deposit library allows and which quietly changes what the library is for.
None of that is why the atlas holds the library. The atlas holds it because a manuscript older than every one of those statutes by more than a thousand years sits in the same collection, and because the institution that ingests a blog on Monday is the institution responsible for keeping that manuscript legible for the next six hundred years. Deposit libraries are where those two timescales meet.