Ultra marathons by definition are distances greater than the 26.2 mile marathon distance. Ultra marathons come in different varieties of distances ranging from 50k to 500k and all kinds in between that. The longer distance you run, the more you will benefit from having a crew and pacers.
The jobs of crewing and pacing are vastly different. Crewing does not require much running if any at all. The job of your crew is to help you get what you need when you come through the aid stations so that you can get out quickly and back onto the course without wasting much time. They might fill your hydration pack for you while you are changing shoes or have your favorite sports drink mixed and waiting for you. These little conveniences add up to a lot of saved time on the course, especially when you are doing a multi-day event. Just think, if you spend an average of 10 minutes at an aid station, after 6 stops you have added an hour onto your race time! Having a crew helps cut that time down significantly and allows you to get back out without wasting precious minutes on the clock. Crewing can be a tiring job, especially if your crew has to chase you around a mountain or difficult terrain. Looped races are the easiest to crew at because you get to see your runner coming through each loop. Point-to-point races require your crew to be mobile at all times and navigate the course to get to the crew/aid stations to help you.
The job of a pacer is to keep you moving at a steady pace towards the end of your distance. For example, if you are running a hundred mile endurance run, many race directors will allow a pacer to join you after mile 60. At this time, you pacer will stay with you for the remainder of your distance. Some runners will trade pacers out if the remaining distance is too great for one person. The job of a pacer is to keep you alert and to keep moving forward. They will remind you to eat and drink and offer emotional support as your trek your way through the remaining miles of your event. A pacer will often keep track of pace and the time left on the clock so they will know when to push you a little harder to make a time cut off or allow you a five minute rest if time allows. For many runners, having a pacer to handle those issues at the last half of the event is comforting. For some runners, having someone to run with makes the distance more fun as well.
Not every runner chooses to have a crew and pacer, and some runners are successful without them. Having a good crew and pacer team can enhance the experience and help you achieve a personal record in your event.