Email is an owned channel: you do not pay an advertising fee every time you want to reach your list. Because the cost is mostly a platform subscription and content production, a healthy list measures an ROI noticeably above the other channels.
The calculation chain: list × sends × open rate × click rate gives the traffic arriving at your site; that traffic's conversion rate and your average order value determine revenue. The two levers for growing the return are list quality (opted-in, active subscribers) and segmentation; bought lists lower your rates and damage your sender reputation at the same time.
