I'm a big advocate of having a wedding planner. In this day and age, with the cost of weddings getting more elaborate and expensive, I believe a bride needs someone on her side who is looking out for every detail on behalf of the bride.
Many brides choose to skip the cost of the planner because "the venue has someone who does that." I'd like to caution brides to take care in this decision. The venue coordinator works for the venue, not for the bride. The venue coordinator will probably not be at the church before, during and after the ceremony, nor will they be there between church and venue during the photography shoots.
I could go on and on but I'm just going to refer you to this wonderful article from the Get Married blog. They list it out perfectly.