As a tool developer I want to know the URL of the article linked to Wikidata item so that I can use a precise URL in my tool
Sitelink data returned e.g. by the GET /entities/items/{item_id} request contains information about the site (its locally configured ID), article's title, and possible item badges.
Knowing the exact URL of the article page requires finding out what URLs are relevant for a particular "site" -- this require making further API calls and to a different API.
Sitelink data should also include a url field that includes the relevant URL -- the same URL that "Action API" wbgetentities returns when providing props=sitelinks/urls parameter, see e.g. https://www.wikidata.org/w/api.php?action=wbgetentities&ids=Q42&props=sitelinks/urls