What is the point of Ozark's flashback episode?

Ozark's flashback episode, "Kaleidoscope", the eighth episode of the series, feels totally out of place in this series.

It looks like the only thing the writers wanted was to explain that Wendy agreed with Marty laundering money for the cartel before he accepted to do so.

However, this is something we already knew!

Indeed, we know this because:

1. She never, ever questions the fact that Marty was laundering money. I was even baffled by the first episode where they didn't show her raging about her husband laundering money on screen when it's kind of needed, and can make a good drama moment if properly handled.

2. She never puts Marty in the "you're responsible for this" situation. That's a kind of a teller.

3. Marty can't lie: when the FBI interrogates him, Wendy comes to the rescue because he just can't lie. More, he's driven to tell the truth, no matter the consequences. Not telling the truth to his wife would have been so out of character for him. The only secret he kept was that he had hired a PI and gotten a video, but that was because there was secrecy from Wendy in the first place, and when confronted, he immediately admitted.

So around the fourth or fifth episode, you naturally come to the conclusion that she's in from the start. Not that she learned it in-between, but that she was in from the start.

And that whole episode leads to the big reval: that both Wendy and Marty agree that Marty should work with the cartel. The rest feels accessory, but the "reveal" falls flat because it's known, acknowledged and processed for a long time already.

The only thing that isn't accessory in this episode is the relation Petty has to that cartel, or at least to drugs. But this could have been scenaristically handled in a total other way that wouldn't need to require such an out-of-place episode.

So, all in all, what was scenaristically the point of making that episode as a flashback episode, potentially skipping better drama or storytelling in the "present"?