In general, people make decisions for emotional reasons, not rational ones. Your job as a copywriter is to evoke the emotions that will persuade your audience.
To do that, you need to know that audience very well, then know your product well. The "magic" of great copywriting happens when you find ways to connect that product with that audience.
One powerful way to do this is to write in the way that your audience talks. If you're writing for lawyers who play golf, use complex sentences and golf references. If you're writing for suburban moms, focus on "me time" and benefits to children. Use emotion in your writing to evoke corresponding emotions in your audience.