BCD maps the decimal numbers 0 through 9 to their four-bit binary equivalents (0000 to 1001), wasting the remaining six states (1010 to 1111) of the four-bit nibble. It simplifies the design of digital displays and financial software because there is a direct one-to-one mapping between the binary data and the human-readable decimal digits. This avoids the rounding errors that can occur when converting standard binary fractions to decimal.
Heard on the job
The real-time clock outputs the time in BCD format, so you have to mask off the upper nibble to get the tens digit.