This phrase started becoming very popular a few years ago and I had a colleague who was particularly fond of using it.
I must admit that when I first heard people using it, I had the same reaction as Rat. It seemed like an irritating banal tautology. After all, if something is, how can it be anything else? But over time, I have come to terms with its use as a statement of resignation about a situation that cannot be changed. It can even be viewed positively as a call to accept the current situation and take action based on it, rather than simply whine about something that cannot be changed.
It is akin to “what will be will be”, though that carries a sense of fatalism, that we have no agency over the future.