This is an interesting question which we answer all the time. You should not hire an investment advisor to give you a perfectly-crafted investment strategy. You should not hire an investment advisor to "watch the markets" for you. We've detailed, in the section regarding passive investing, that investing should be rather simple.
But you should hire an investment advisor for the following reasons:
First, most people are emotional about their money. When they see the periodic volatility of the markets, they get either excited or scared, and they let those emotions drive decisions - BAD decisions. Most people buy AFTER the market has gone up, when it's expensive, and sell AFTER the market has gone down, when it's selling at a discount. This is a BUY HIGH, SELL LOW philosophy, and this is exactly the opposite of what investors should do. Of course, when the markets are calm, it's quite easy to say you will not engage in this type of bad behavior. Yet when things go wrong, well-laid plans get thrown out the window. SO, we recommend hiring an advisor to keep you disciplined.
Second, a good financial advisor will manage your investments with taxes in mind, making sure your investments are placed in the right accounts (taxable vs. IRAs vs. Roth IRAs), taking only long-term capital gains, and otherwise mitigating your tax burden.
Third, a good financial advisor will create that plan with your input, make sure your progressing toward your goals, and refine and adjust the plan along the way.