Why is good attendance important from the employee and the supervisors point of view?

Generally an employee does one or more tasks as a part of what needs to be done to complete the work for which a company charges their clients. Frequently one person's job must be completed before other people can begin their jobs and if they are not there the work schedule gets screwed up, it's not done on time, and the client is not happy. The work flow in general is based on everyone doing their job every day and, again, schedules get screwed up when people are not there. If a company can't rely on someone being at their job, they will probably look for someone else who is reliable about showing up so work can be done by the time it is promised.