Assembler : why BCD exists?

BCD arithmetic is useful for exact decimal calculations, which is often a requirement for financial applications, accountancy, etc. It also makes things like multiplying/dividing by powers of 10 easier. These days there are better alternatives.

There's a good Wikipedia article which discusses the pro and cons.

BCD is useful at the very low end of the electronics spectrum, when the value in a register is displayed by some output device. For example, say you have a calculator with a number of seven-segment displays that show a number. It is convenient if each display is controlled by separate bits.

It may seem implausible that a modern x86 processor would be used in a device with these kinds of displays, but x86 goes back a long way, and the ISA maintains a great deal of backward compatibility.