Why would you use an assignment in a condition?

Question

In many languages, assignments are legal in conditions. I never understood the reason behind this. Why would you write:

if (var1 = var2) {
  ...
}

instead of:

var1 = var2;
if (var1) {
  ...
}

?

Answer 1

It's more useful for loops than if statements.

while(var = GetNext())
{
  ...do something with 'var' 
}

Which would otherwise have to be written

var = GetNext();
while(var)
{
  ...do something
  var = GetNext();
}