People buy when they feel that they have good reasons to do so. So, you need a strong value proposition (=great reasons for buying what you sell). If you don’t have it, you can’t be able to give people good reasons for buying. If you don’t know what—specifically—would make people see value in your offer, how could your email marketing (or any marketing) be effective?
Minimal risk: Some top marketers expect to burn through $10,000 of advertising budget before even knowing if they can make a campaign profitable. (Nope, that’s not an exaggeration. But I’d usually aim for a few hundred dollars—not thousands.) With email marketing, the costs are much, much lower. A simple email marketing software like AWeber is very affordable even if your business isn’t a huge success, yet. And even if you go with something advanced like Infusionsoft, you’re still far away from the kinds of investments many other marketing tactics require. Sure, email marketing and advertising aren’t directly comparable (advertising is mainly used to reach new prospects). But you get the point.
When you're using email as a marketing tool, you're working on a pretty personal basis. This isn't ‘drive-by' stuff -you're directly targeting customers from their inboxes. There's a huge amount of potential which comes with this kind of marketing... but you'll only reap that potential if you REALLY understand both the marketplace you're operating from, and the audience you're talking to.