A coordinator CAN'T prevent all the issues and emergencies from occurring. We CAN have solutions for most of the common problems that arise.
A coordinator ISN'T psychic. Since you have planned your wedding, your coordinator will need to know all of the smallest details to keep your wedding running smoothly. A coordinator IS good at juggling all the details and timing involved in a wedding.
A coordinator CAN assist other vendors throughout the day, e.g. caterer, DJ or photographer. A coordinator CAN'T do this if she is also supposed to be handling other tasks at the same time. A good coordinator will also point this out to you while creating or reviewing the timeline.
A coordinator IS excited to see all of your idea come together on your wedding day. A coordinator ISN'T a fairy godmother who can create extra centerpieces or decor if it wasn't planned.
Most coordinators WILL work long hours the day of your wedding and spend hours on their feet. A coordinator WON'T know that you have planned for them to tear down and pack everything unless you have talked about this before. Please communicate this beforehand so the coordinator can plan accordingly.
A day of coordinator can be your best investment for your wedding. Just have realistic expectations of what they can and can't do for you and be sure to communicate what you need.