I'll stand to be corrected but I think it improves the colour rendering and the efficiency by reflecting the UV portion of the emitted spectrum back at the filament. In the earlier days of this type of lamp they also used to be more prone to exploding when the filament failed so the glass cover shield also offered some user protection if this happened.
I'm not sure about the temperature difference but I don't think there would be one. There's a direct relationship between filament temperature and the colour of the light, the colour is given as a temperature in Kelvin. Warm white is around 2500K upward and as the temp increases above 4500K it becomes cool white. Eventually as you get hotter you get 'daylight' colour around 7000K so to answer Martinco's question, if the filament temp was higher then the light colour would be different.
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