Saying no is an important skill when you are at the workplace. There are many times where people are going to ask you for favors, to stay late, or to do things that they don't want to do themselves. If you are going to survive in the workplace, you are going to want to be able to say no, and for a variety of good reasons.
The first reason is that people don't like to work, and so people are going to be pawning off as much work as possible. While it isn't always fair, and sometimes the boss will intervene, it makes sense to be able to say no, and avoid the workload. You don't want to get too stressed with your workload, especially since your own workload can be really heavy to begin with.
As well as not being a pushover at work, there are other good reasons to say no. Saying no means that you are strong enough to know when people are trying to pull something on you, and it actually gains you respect in the workplace. When people see that you have the skill to say no, you are going to be seen as someone who might be able to think for themselves.
This is a good thing because it means that you are going to place yourself on a pedestal for the moment. When the boss sees someone who is able to say no, they see someone who might even be able to say no to the boss. Believe it or not, but the boss is actually a person too, and doesn't always want to be surrounded by yes men and yes women.
By being able to say no, you are showing that you can be discriminating in your decision making. Typically discrimination isn't something you like to see at the workplace, but when it comes to making choices about your life, being a little discriminating is OK. When you are able to discern for yourself what the best course of action is for you, it gives you tremendous control over your career.
Saying no is an important skill to have in your working years. Saying no means you won't just be a pushover at the office, and you won't just do what others want you to. The boss will notice and take note of the fact that you can think for yourself, and the fact that you can make a critical decision. Saying no is a great skill to have if you can muster up the courage to use it.