Why Initialization Lists in Constructors?

The old method of initializing variables, was as follows:

class monkey{
  banananas b; //
yes I know how to spell banana, I just forget when to stop.     
  tree *t;
  monkey(banananas x){
     b = x;
     t = new tree();

Nowadays you are told to use Initialization Lists in this manner:

monkey(banananas x):b(x),t(new tree()){}

Where it looks like b(x),t(new tree())  is the same as b = x; t = new tree();

It is not.

t = new tree();

causes a temporary anonymous variable to be created.

anonymous variable = new tree();


t = anonymous variable;  // using the assignment overload.

finally destruct anonymous variable.

Suppose that tree allocates 1 M of memory and then opens a file reads it.

Upon calling the destructor, the file must be closed and the memory free'd.