When it is extractive, convergent search: you know what you want up front. You’re trying to get mentions of a specific relationship, entity, answer etc.
There’s also browsing and sense-making (divergent) which happens before you know what to search for really
When the corpus has high quality information
When the domain is messy, unstructured and hard to navigate (like the web)
When you’re reviewing the literature along a very specific dimension, keyword or method
But generally, when a knowledge network has a big social component, context-free search is less valuable. Users will orient around social structures