What you are drawn towards matters.
Interests can influence the kinds of activities, subjects and work environments that individuals find engaging. Understanding interests can open new possibilities for career exploration.
The short answer
People are unreliable witnesses to their own interests. Asked directly, they name what sounds respectable or what they were once praised for. An interest inventory works around that by asking about activities rather than ambitions, and by comparing the pattern of answers against people already working in different fields. What comes back is often mildly surprising, and it is usually the most useful part of a report.
What people ask about this
How do you work out what interests someone?
Interests inferred from activities, not from stated ambitions.
What are those interests compared with?
Patterns compared with people already in the work.
What do I get at the end of it?
A widened list of fields worth looking at.