Having a photographer is super important for any special event as they just fly by. Especially wedding days when emotions are high, you'll find it difficult to remember every detail so photos are important to help jog your memory for all the little moments.
If you've never been married before and you're not really sure how a wedding runs then please take my word that every second goes by so quickly. One moment you are getting ready and the next you're sitting down for the wedding breakfast (and if you're anything like me, those few minutes will drag whilst you wait to be fed..). Then you'll be having your first dance and after that you'll probably be too drunk for the rest of the evening to remember anything anyway. So, having someone there to capture all those moments between the blurs for you helps.
It is also important to note that you must stick to your schedule where possible. If you would like some photos pre-ceremony e.g. the bride with the bridesmaids/ groom with the groomsmen then you must allow at least 30 minutes before it starts to get the photos. There is usually around a 1hr 30min slot to get all of the photos you've requested in between the ceremony and wedding breakfast, which isn't that long in the grand scheme of things... especially when you may also want to spend some time with your guests/ eat lots of canapes and you'll also want some shots of the room before everyone sits down.
Planning ahead for any big event like a wedding is really important... However, as long as you both do your best to stick to the schedule then all will be fine!
Before every wedding I'll send you a questionnaire for you to fill out which helps me see when I have the chance to get the photos you've requested. I'll then send you a rough schedule for us to try and stick to... which really helps!