Every DTC operator knows this. You throw money at Meta and Google and hope. One Tuesday your CAC is $35. The next Tuesday it's $80. Same creative, same audience, same offer. The auction shifted. You shrug. You burn budget anyway because there's nowhere else to go at scale.
Then you look at your email program. You send a campaign and you know what you'll make. Maybe not the exact number, but a tight range. Open rates, click rates, conversion rates, revenue per send. You control the list, the timing, the offer, the copy. Email is predictable because you own the inputs.
That's the asymmetry that breaks brands. Half their P&L sits on a channel they don't control, and the other half on a channel they do. The unpredictable half eats the predictable half's margin every month. Acquisition needs its Klaviyo moment.