I dont know if there is any legal obligation to it. But the application depends on the following:
1. Sponsor Eligibility
2. Applicant's medicals
3. Applicant's Criminality Check
4. Applicant's Security Check
5. Relationship Genuinity
If they find anything that needs to be clarified, they can do it by asking for docs or asking people to come in for interview.
Additional docs are GENERALLY for the first 4 items in the above list. If they are asking for item 5, it is generally updated chat history, photos, proof of relationship letters from friends/relatives.
But if there is something that needs to be clarified by direct meeting, then the interview will be called for. Again, this inference is based on what I have seen so far in the forum for the past 8 months.
Mari, these are only my observations and I hope it clarifies your question.
Senior forum members, please feel free to chime in here.