One of the great things about Illinois workers' compensation law is that it's a no fault law. That means you don't need to prove that your employer was negligent to win your case. So if your back went out on you because you used bad form when lifting or if you new the floor was just waxed, but walked on it anyway, you can still get work comp benefits even though you are partially at fault.
As a result, in most cases you don't need a witness to win a case as your testimony is usually all you need. But does that mean you never need one?
The answer is that if a witness will help your case, they should probably be at the trial. This is especially true if your employer is disputing your version of how you got hurt.
So if a co-worker can confirm that a floor was wet or that you lift a certain amount of weight or that you told them that you felt pain while doing a work activity, it can help.
Even video testimony can help. In most work comp trials the only video that is shown is either surveillance of you doing activities you said you can't do or a video demonstration about what your job duties are. In a recent successful case though involving a CTA bus driver, the worker won his case because the dash cam showed the impact he suffered after going through a crash.
The video evidence was used to push back against the company doctor who said he was fine.
Bottom line is that you don't have to have a witness or evidence and in many cases there isn't one. But you should at least consider having one and if your attorney isn't asking if there is one early on in the case then they aren't doing their job.
We tell people to keep a journal of what happened throughout the case and in the beginning it should certainly include who you told, where you were when you told them, what you said, etc. It's not a death knell to your case if you don't, but it certainly could be the difference between winning and losing in the end.