Query Federation is an important tool in providing intranets and portals in several scenarios:
Data residency, information export, and contract restrictions may make centralized indexing impossible
Organizational control, varying requirements, or logistical needs may make separate search implementations necessary or desirable
External content such as subscription content, news sources, web search engines, or large-scale research repositories may not allow crawling, or it may be prohibitively expensive to crawl and index
These situations require a unified view but do not allow for all content to be in a single index. To address them, a capability to query against multiple indexes and collect the results is necessary.
The OOTB query federation capability (called Remote Result Sources) is a good mechanism, however, it does not provide what most users want: a single set of results and refiners across all systems.