It is always important for one to be held accountable to the work that they are assigned to and also the hours and days because simply put, that is their duty. According to most work contracts, employees are supposed to complete any work assigned to them and cover their days and number of hours of their in their work place.
Because other people, such as your coworkers & clients, are counting on you to be there. In the school I've been working in this past year, one of the aides has gotten extremely spotty about her attendance, & doesn't bother trying to get a substitute half of the time. It has been very hard on her coworkers & the students she's assigned to work with, not knowing if or when she'll be there.