Most day of coordinators are month of coordinators. It truly is entirely up to you if you want a coordinator. There are ways for you to lay out a detailed timeline and do all the planning without using one. Make sure they fit into your budget, because they can be quite expensive.
A coordinator provided by the venue only focuses on the venue, and most don’t care about you. A day/month of coordinator works behind the scenes to make the day run smoothly so your guests don’t have to work. They take care of situations such as a vendor not showing up (stuck in traffic or double booked) so that you and your guests don’t stress. They set up and tear down as well.
A day of coordinator is vital no matter how few guests you have.