Well, first off I do these changes for me noone else, I just post what I do here so that I will remember it.
The reason is that I've noticed that whenI work logn houres with big screens my eyes does not get enough rest. And the strain of white against black background is very big.
So I have tried to change this to get a not white but lighter background and still with good kontrast, I have not succeeded that well at the contrast. Maby some colour professional can help me there ;).
However this is the main reason.
Now why would I not set this when the session is created?
I have a number of reasons for this.. mainly because I do not use putty sessions. I use commandline or write all on the fly. I often work where I will remove the program afterwords in unsecure places where you do not want to save sessiondata. Also when I use commandline I want UTF and I want the interface to always look the same.
This is why.
/Jacob