Something I always wondered

[darklife]

Why are some tubes painted gray or black inside on the glass?
I never understood the reasoning behind this. Not to confuse with black plates, I realize black plates tend to handle more heat unless I am wrong about that.
But a lot of old tubes I have had would have half the top darked out so you couldn't see it glow or the internals.
Is there a purpose for this other than annoying me when I want to see them glow and look at their internal structure?
On this note.. I absolutely hate metal enclosed tubes.