You've probably heard of athletes finishing a game on a broken leg or a broken ankle. After the game, they can barely move and need surgery, but they were running around before that.
Adrenaline is incredible. It can power people to do things they'd never do without that rush. It can make serious pain feel minor and those who are "in the zone" may be able to function as normal, even with serious injuries.
Car accidents are far different from sporting events, but they can cause similar injuries and a similar rush of adrenaline. In some cases, people put off getting care and going to the emergency room because they think they're fine.
For instance, you feel some neck pain after the crash, but it's not bad. You think it's just from having your neck snap forward. You rub at the pain as you talk to police.
The next morning, though, you wake up and you can't turn your head. When you get to the emergency room, they tell you that you have a broken neck.
You can see how problematic this can be when seeking financial compensation for your injuries. If you assume you were not hurt, that doesn't necessarily mean you're right. The adrenaline may just be masking the pain, but the true extent of your injuries will not become clear for some time.
This is why some people suggest erring on the side of going to the ER if you're hurt at all. In some senses, save for minor bumps and bruises, it's better to be cautious and have a doctor confirm the extent of your car accident injuries.
Source: Vital Record, "You Asked: To go to the ER or not? What to do after a car accident," Dominic Hernandez, accessed Jan. 18, 2018