Make your main point the very first thing people read. People are busy. They may or may not have time to pay attention to you. Therefore, you need to say what you need to say straight away. Don't string 'em along, get it out and let them immediately judge if they want to hear more.

Stating your message up front is also useful if you're making a long argument. If people know where they're going they're much more likely to follow. So start out with your conclusion, your main point, the reason you're writing. The first sentence is a good place.

Note to grammarians: yes, I know it's "lede", but let's not confuse people more than we need to