Why/when should I use BigDecimal instead of double for numerical calculations?

Precision and accuracy. double uses a fixed-size data format so it can't always be precise or accurate in its representation of numbers. On the other hand, the BigDecimal class will use as much space as is necessary and uses computational algorithms that will be both precise and accurate for any number.

If you're dealing with any sort of numerical use where spot on precision and accuracy are critical then you really need to use BigDecimal rather than double.