This all comes back to a simple fact. You have to do something for a long time before you get really, really good at it. If you know who Malcolm Gladwell is, you should know about the 10,000 hour rule. Not only will you be "not great" at something the first time you try it, you'll probably be terrible; at least looking back on it ten years later.
So what do we, as a society, do about that? We try to invent shortcuts to greatness. We build tools that supposedly will allow you to reach mastery level in a few, short hours! Excellent! Sign me up. But there's a problem. There is no shortcut. There really is no substitute for experience. The great thing about experience is that it will sometimes be incredibly rewarding, and other times it will totally kick your ass. But you're always learning.
That brings me to the topic at hand: Hiring a professional. Use this simple formula when you need to think about hiring a professional...
Ask yourself, "How important is it that I have a great outcome?"
Ask yourself, "How many times have I done this before to guarantee a great outcome?"
Ask yourself, "If I have a bad outcome, what effect will that have on things?"
Ask yourself, "What is the value to me if I have a really great outcome?"
Take those answers and divide by seven. If you end up with a number higher than 10, you should do it yourself. If you have no idea what I am talking about, you should hire a professional.
True professionals with experience will get you the results you need. There's no guesswork, there are no excuses. Can you build your own website? Sure! There are hundreds of shortcut tools out there that let you build a "professional looking" website in just a few hours. Will it bring customers to your door? Will it magically make you a search engine optimization expert overnight? Good question. (Hint: No)
Make your own business cards, design your own logo, create your own email campaigns. The list goes on and on. Maybe you will get something good, maybe not. But I will guarantee that rarely will you ever get something awesome. As a professional, my goal is to deliver something awesome every day. Probably after about ten years I was able to get a little closer to doing that. After twenty, even closer. After thirty (that's right - I'm old) closer yet.
If your business depends on standing out and differentiating yourself in a crowded market, you probably won't be able to do that with a template website, a clipart logo, and a "canned" TV commercial. If your budget allows, look for help from that person who already has the 10,000 hours under their belt. After all, why settle for a macaroni necklace when you can have the Sistine Chapel?