Why do "good" lights not sit flush on glass top?

[Passatryde]

It seems almost everything I see sits up above the glass on the back of the tank - why? I would think the higher it sits above the glass part on the back of the tank the less likely the light is to spread throughout the whole tank

 

[Coursair]

Sometimes it's to diffuse heat. Don't want to crack your glass.
Sometimes it may be due to the popularity of rimless open top tanks.

 

[rkilling1]

It's also nice to have the light off of the tank so you have plenty of room to work with the plants.

 

[mohican]

I actually contacted Current about the nova about this very issue and I was told the "loss of light" is actually very minimal. Coursair had it right tho, I think the HO bulbs run hotter than NO bulbs which is why most HO fixtures are elevated.