comma after 'often' [adverb] / with two introductory clauses

[Julianus]

Hello.

1.a. Very often, when a society adopts a new food without the food culture surrounding it, as happened when corn first came to Europe, Africa, and Asia, people get sick.
(from Korean university entrance exam)

What reasons do the successive commas after adverbial phrase 'Very often', adverbial clause 'when ~ surrounding it' and 'as happened ~ Asia' have?
Are they for just distinction between several adverbial phrase(clause)?

Thank you always~.

 

[MuttQuad]

One use of commas is to indicate pauses for the reader. that is what your first commas are doing. Rules for their use in this way are very subjective. The serial commas between the named nations, however, are just that: serial commas.