Do you have a job that requires a lot of your time? Or do you just have a lot on your plate in general? A wedding planner will help you schedule meetings with vendors (or even attend the meetings for you!), pick out linens and invitations, and take care of the other tasks that you just don't have time for.
Since wedding planners are professionals, they know the industry much better than the average person. With that in mind, they know exactly where to go to get the services and products that you want, which will cut down on the time you spend searching for what you like.