key levelling -- crown?

[Charles Neuman]

I just learned that when levelling white keys, some people put a small 
amount of crown in the key heights, maybe 1/32". What's the point of it? 
And does 1/32" really make any difference?

The only reason I can think of now is that it might have some subtle 
visual effect. If it's some ergonomic thing, I would think it would be 
much more than 1/32".

Charles Neuman