Compared to a casual Western phrase like “So, what's the deal?”, `究竟` carries more weight. It's not typically used to start a light conversation. Instead, it's deployed when a situation is already confusing, frustrating, or mysterious. For example, if a friend who was excited about a trip suddenly cancels without explanation, you wouldn't just ask “Why did you cancel?” (你为什么取消了?). You'd more likely ask, “Why on earth did you cancel?” (你究竟为什么取消了?), signaling your need for the true, underlying reason. This reflects a practical directness in getting to the heart of a matter.