If the strap is not right, your helmet can slip to the side or to the back. Then your bare head can hit the road, ouch! The pavement is very, very hard. Being careful and not crashing is the best way - that's better than crashing in a helmet!
The helmet only covers your head. So you need to learn the rules of the road and ride that way. But even the best riders crash.
If you hurt your brain it can change you. You may not be able to read this page, or play video games, or talk, or run, or tie your own shoes or even feed yourself. Some people do not wear helmets, do not let that stop you. You need one when you ride your bike or skateboard; they do too, but they just don't know it yet.