Because the index into the arrays is the identifier for an entity, subsystems can keep attributes related to the entity in their own private arrays. They don't need to modify record definition or use some complicated attribute extension mechanism or associative lookup mechanism. Their access will be just as efficient as other attributes.

It is indeed about row-oriented vs column-oriented, but the distinction isn't merely "storage layout", like how you'd choose to the order of dimensions in a multidimensional array.

Rather, it affects how the software is designed, developed and debugged. The referenced PDF with a defence of the approach by Adam Rosenberg is quite interesting: http://www.the-adam.com/adam/rantrave/st02.pdf - particularly pages 16 and onwards until you get the point.

Adam Rosenberg has some real points. The billions of dollars lost to insecure programs written in idiomatic C style wouldn't have happened, at least not in the same way, with the parallel array approach. For my part though, I think that parallel arrays represent a local maximum. I found Adam fairly convincing, and I think the approach is probably superior to pointer soup for programs that can fit inside a single person's head. But I don't think the approach scales; you need better encapsulation and composition tools to scale up, and those come with more indirections.