Wikidata is a semantic web thing to collect facts about the world in a machine readable way (e.g. queryable via https://query.wikidata.org )
Generally speaking, wikidata is more broad than wikipedia.
Comes in handy when updating the interwiki language links, or being able to automatically fill out those infoboxes.
Wikipedia is actual encyclopedia and useful
Wikidata is a weird idea to create a weird “semantic web” - a hierarchy of everything that ever existed or will exist - and it’s about as useful and easy to understand as previous experiments with semantic web.
Wikimedia seems to double down on this instead of letting it die; I really don’t understand the point, I don’t think that it’s useful and it’s really hard to edit, as a editor.
Interesting thing is - it’s CC0 instead of creative commons for some reason. Their argument is that you cannot copyright a fact. I donno.
The audience is quite different, and it may very well not be useful to you, and that is ok, but wikidata clearly has an audience and seems to be on an upswing. Its certainly doing better than pretty much all the other sister projects.
As an aside, politically wikidata is really WM-DE's baby, so its really more them doubling down and WMF along for the ride, but of course its all very intertwined.
It’s definitely not the least useful of the wiki projects - that award definitely goes to WikiNews - but at least WikiNews is not intertwined with everything on wikipedia.