This pattern conveys a sense of inevitability or obligation: you do something because circumstances force you, even if you'd rather not. It's stronger than なければならない, which simply states necessity. ざる is an old negative auxiliary, so the whole expression has a formal, written tone. In conversation, you'd often hear it in news reports, business settings, or when someone explains a difficult decision. For example, a manager might say they had to cut salaries because the company was failing. The pattern implies reluctance: it's not that you want to do it, but you have no alternative. It can also be used for logical conclusions: if the evidence points that way, you can't help but believe it.