Why does this code exist? What are you working around? What are the assumptions? What limitations? If it's complicated enough that I'm only understanding it

Why does this code exist? What are you working around? What are the assumptions? What limitations? If it's complicated enough that I'm only understanding it because of the context of the last week, we need the comments. Anything that can speed the reverse engineering in 6 months when it breaks is helpful, because then you can quickly decide that we got different input or if we missed an edge case or whatever.