In true ELI5 fashion, you remember how when you were 5, how all of the counter tops were out of your reach? And to get your toothbrush or something, you’d have to go find a foot stool or chair to stand on? That’s because the house was designed for fully grown adults. Well, if you are the height of a 5 year old in real life, there’s no reason why you have to be the height of a 5 year old in virtual reality. Everything in virtual reality should be equally accessible to everyone regardless of their height in real life.
If you don’t calibrate and account for the players height, then a 5 year old in real life is going to have the experience of a 5 year old in virtual reality. That magical key you placed on the table to unlock the chest will be out of reach (and probably not visible).
On the other extreme, imagine that you are playing as a 10 meter giant in VR. In real life, your eye height might be at 1.5m, but in VR it needs to be at 9.7m because you’re a giant. If you assume that the player is always 1.5m in height and you boost the camera position exactly by (9.7-1.5) = 8.2m, but the player is really 1.4m, then the boosted eye height is going to be 10cm off and the player is going to be looking through the nostrils of the giant rather than the eyes of the giant. And if you’re a five year old, you’re looking through the giants neck. Now, if other characters try to make eye contact with you, it’s going to be looking kinda weird because they’re looking at something way above your head, or looking at something on your chin.
As you can see, it is a lot easier for the designer to design environments for a known character baseline and not worry about variations in players physical heights, shapes, sizes and limitations.