You clearly don't know what all a coach actually does, so here is what track coaches do and why they are not pointless:
-As you mentioned they plan workouts. Good coaches actually understand the physiology behind training and put a lot of effort into write good training plans designed to get athletes to peak at the right time. Yes, you could read a book and possibly get good results, but a good coach will write plans customized for their specific athletes and circumstances which you can't get from a book. Good coaches also have developed the art of coaching and know which adjustments to make and when. Majority of high school and college track athletes do not have an adequate background to be able to do this effectively. Most of those that do have some understanding of this, learned it from their coaches.
-Coaches have to motivate their teams to come together and perform their best when it counts and get them in that right mind set. This includes both helping pick people up when they're down and also channel energy when people might be overpsyched. They also help manage team relations and personal issues on the team. The right approach is different for each person.
-Coaches manage the team. This includes strategically designing the schedule, gathering meet information, signing up for meets, paying entry fees, communicating information to the team and in some cases parents as well, some will manage team websites, manage the captains, awards banquets, plan meal stops or order/purchase food for meets that require that, book travel arrangements including buses and hotels when required, communicate with athletic trainers and athletic directors, organize and manage fundraising in some cases, hire and manage assistant coaches, manage equipment and uniform inventory and order new stuff when required (which often includes purchase forms and approvals etc)
-Coaches manage skill sessions as well. In track and field there is more than just running. Field events need a well trained coach to plan both workouts and skill sessions and give feedback on the athlete's technique. This requires being able to spot what is right or wrong in each performance and give feedback in an effective way that the athletes will be able to use to make adjustments. They have to be clear without overwhelming them with too much feedback at once. This is harder than it sounds and takes practice. Even for running events, coaches should be giving technique feedback or incorporating some drills and stuff to work on running form, starts for sprinting, or relay handoffs.
-If your team does weight lifting, so athletic associations or schools have rules that a certified weight lifting coach must be present to supervise. In some cases, this may be your coach with the certification allowing the team to lift at at that time.
-Coaches make strategic game time decisions. This includes things like deciding the best relay order and which athletes to use in the relays, deciding which athletes to put in which events to score the more points, and giving feedback during events to help get the best out of athletes. Having a second set of eyes to tell you someone is sitting on you waiting to kick or closing hard on you can be helpful when you're racing and can't see behind you.
-Good coaches serve as mentors for athletes that help teach them things that go beyond sports and prepare them for later in life. In the cases of youth sports, the coaches also provide supervision to athletes when parents aren't there.
-Some high school coaches will help walk their athletes who wish to compete in college through the recruiting process. In the case of college coaches, recruiting is 50% or more of their job and the reason you have a team each year and the coach keeps their job. Recruiting in itself involves identifying prospective recruits and getting their contact info, sending thousands of emails or letters to prospects, following up with interested recruits, making lots of phone calls to high schoolers, going to high school meets, communicating with admissions, assisting with admissions process, marketing team to recruits, and organizing recruit visits.
All of this stuff takes a lot of time and effort and requires knowledge and specific skills to do it well and thus why coaches are needed. Yes, you can coach yourself and some people will do okay that way, but having a coach certainly helps and track coaches do as much work as any other coach.