What's the point of setting Height and Width in NaN?

[SKProCH]

I've seen several examples where the Height and Width values are set to NaN and the behavior changes.
What exactly do these values do?

[maxkatz6]

NaN - default value for Width and Height.
If these properties had different value set (for instance from styles), and you want to set it back to default, you need to use NaN.
It's also an equivalent of Auto value from WPF.