Will the company you work for miss you when you move on?
You might think that you are a valuable designer because you get through all your work every day, produce good designs and always deliver on time.
The questions is, are you the only designer that can get through that much work a day, produce good designs and always deliver on time?
In today’s economy there is probably someone who can do what you do and who will do it for a smaller salary.
To be a irreplaceable, you need to be more than a good designer. You need to find ways to add value. Become THE BEST at something and apply that knowledge to benefit the company.
The same applies to freelancers. Give your clients a reason to come back. If you do only what is expected, you are not doing enough. If the quality of your work is acceptable, it is time to start aiming higher.