OK I see (roughly) HOW it works if a variable is declared volatile, but what is it good for, then?

Ok, visualize this one:

if (Ralph == 1)
{
  ....
}
else if (Ralph == 2)
{
  ....
}
// here an ISR changes Ralph from 4 to 1
else if (Ralph == 3)
{
  ....
}

else if (Ralph == 4)
{
  ....
}

no amount of applying volatile will help here, the only possible way is to use a local copy of Ralph.

now, visualize this one:

if (flag)
{
  ....
}

the compiler is fully justified in copying flag to a register and testing it there forever if flag is not declared volatile.

SO 'volatile' does good when you know when to use it and when not

Erik