Y delay after analogread()?

Hi folks,

often I find Code that looks like this:

accelerometer[i] = analogRead(i); 
// delay to allow analog-to-digital converter to settle: 

I recognized that this delay is used sometimes after digital and sometimes after analog input.
My question is why?
I think that the analog-to-digital-conversion must be finished after analogRead was executed.
And when, why so long?
In the atmel manuals it is written that the maximum amount of time needed for analog conversion is about 320 micro seconds (depending on processor).

Thanks for your help!