Giants like Google, Amazon, and Facebook have imposed countless rules that limit what you can and can’t do to promote your business. If you repeat content, you’re in trouble. If you “keyword stuff,” you’re in trouble. If you backlink to shady sources, you’re in trouble. If you violate their rules, they penalize you by not displaying your content, moving you down on their results, or banning you entirely.
The most obvious reason to build your email list is to maximize the value of your customers and your company revenue. However, it's also important to remember that your email list is a company asset. In the event that you want to sell your company, a high-quality email list can increase your company's overall valuation. Additionally, a high-performing email list can give your company additional revenue opportunities by giving you the chance to sell advertising space in your email products or send sponsored emails on behalf of partners or advertisers. You're not just building your marketing channel when you build email list. You're also building your company's total value. That's why it's worth your time to build a quality email list over time.
For effective and actionable results, you should do A/B tests, which test two (or more) different approaches. For example, you could send half of your list one subject line, and an alternate subject line to the other half. You could also A/B test the two approaches with a small subset of your list, then send the winning headline to the rest of the list.