whats the purpose of MH bulb burn in time?

[bekindtoyurbuds]

Whats the purpose of the burn in time for an MH bulb?
I have heard that it is about 100 hours before the bulb "burns in"

What does the burn in time do to the bulb?

Does it change the Kelvin rating of the bulb? (meaning that its not at the right Kelvin right out of the box?)

Does the bulb get brighter?

Does it change the spectrum?

What does it do for the bulb?

Please elaborate when answering, or maybe you guys can post some links.

Thanks