When it comes to email and SMS campaigns, a volume baseline generally needs to be met to generate, at least, similar results to the previous month.
If you fall significantly short on the volume of deliveries, your conversion rates need to work extremely hard to make up the difference.
For example, if last month you had 400,000 total campaign deliveries and this month you ended up with half of that (200,000), then just to stay even month-over-month on campaign revenue, you'd have to have double the sales conversion rate (or AOV).
Driving up sales conversion rates is certainly possible over time, especially with effort put into testing/segmentation and also automated flows. But to double campaign sales conversions immediately across the whole set of campaigns is rarely a one-month job. And even then, in this scenario, you'd just be staying even.
By using volume of deliveries as a parameter target, you make sure you are planning enough campaign frequency to enough of the list so that the likelihood of hitting campaign revenue targets is high.
A side benefit is you can also keep better tabs on your expected ESP costs and scale up or down accordingly, as most charge based on volumes of deliveries. This comes into play even more with SMS or MMS deliveries, which tend to have a higher cost-per-sent message attached to them compared to email.
Hopefully you're convinced of the importance of using email delivery volume as a core target to structure your campaign planning around, let's dive into the nuances of how to back out the exact volume targets.
Diminishing Marginal Returns per Delivery
All else being equal, if you keep seeing great trends in said metrics, you can keep increasing the volume of deliveries. However, there inevitably will reach a point of diminishing returns, at which point you'll need to dial back delivery volume increase and reassess relative to the changes in the metrics.