I have to say I use to think that way, why do I want to spend money on something I can do myself. After all we moms are super women right? We can do it all! After my own daughters weddings and working with others that had similar beliefs, I know the why.
Many of people over extend themselves and/or dear friends and families expecting they can do the detail work and oversee the day to save money. However, at the end of the day they then realize they missed "the show". Those of us that saved the money missed being present in the event, the joy and the conversations and sights and scenes of the day. You cannot get it back, yes there were at pictures, and perhaps a video but that does not take the place of the priceless feelings of the day spent being present. There is much to do to make this day happen.
You may be very organized and don't feel you need help all long the way but do yourself and your friends and family a favor and at minimum hire a "Day of " Coordinator (DOC). There are often many small details that you may miss due to your being too close to the subject at hand. Your DOC has the expertise in carrying out the timeline and assuring all the hard work you have put into planning comes to life in the manner you desire. But it takes more than a day.
Day of Coordination is more like Month of because they will begin their work with you 4-6 weeks prior to the wedding gathering information on the vendors you have chosen, contacting them and making certain everyone is on the same page.
So when you think WHY, stop and say WHY would I want to miss this most important day.