What is the difference between Find and Search?

I read the manual and it sort of explains plainly what the difference is.

From what I can see, the Search field is the Window's internal search function. Opus's Find feature, I assume, uses its own type of search engine. While the Search field can search in metadata fields, the Find function cannot. Is that correct?

But, the find function, you can use filters.

Is there a reason why the two aren't simply combined?

Both can both search metadata. They aren't combined because some people like one and some like the other, and some like both for different types of things.