Why are you hosting an event? What are your desired end results? Big words like marketing and branding come into play here. Event strategists define your objectives. You achieve your goals.
Are you overpaying for your venue? Did you get the best deal on centerpieces? What matches orange? Event planners handle all the logistics. You are ready "when the lights go on."
Who is going to greet your guests if you are meeting the vendors, working the registration table and trying to find tape for a broken shoe? Event managers control the "behind the scenes" parts of an event. You shine as the gracious host.