A useful story telling device like an aside, especially given it can point to things in data space on a chart. The contents of an annotation is a normal cell, so you can embed drawings or programs in them, but by default its markdown. Also useful for collaboration, so people can accurately identify content that needs revision without affecting the primary content. (works on Notebook 2.0 too).
This unblocks a really great workflow where you can annotate a draft in place without have to switch around apps or scroll and lose focus on proof-reading. The fact it is a pointer saves a lot of typing and mental alignment to describe where you are in the doc, and because it is out-of-band from the main content it leaves the corpus in it original state. Really productive for proof-reading
Is the notebook published?
Annotate yes, the optical localization no but its the next in this series realtime optical positioning / Tom Larkworthy | Observable