It emphasizes the excessiveness of the spending, implying that the amount spent is much more than what would be considered reasonable or necessary.
Here are some examples of how the idiom can be used:
* "My sister spent a fortune on her wedding dress." This implies that the dress was very expensive, perhaps much more than the average wedding dress.
* "They spent a fortune renovating their kitchen." This suggests that the renovation was very costly, perhaps involving high-end materials and appliances.
* "He spent a fortune on his new car." This means that the car was very expensive, likely a luxury model.
It's important to note that "fortune" is a relative term. What is considered a fortune for one person might not be for another.