I know BCD is like more intuitive datatype if you don't know binary. But I don't know why to use this encoding, its like don't makes a lot of sense since its waste representation in 4bits (when representation is bigger than 9).
Also I think x86 only supports adds and subs directly (you can convert them via FPU).
Its possible that this comes from old machines, or other architectures?
BCD exists in modern x86 CPU's since it was in the original 8086 processor, and all x86 CPU's are 8086 compatible. BCD operations in x86 were used to support business applications way back then. BCD support in the processor itself isn't really used anymore.
Note that BCD is an exact representation of decimal numbers, which floating point is not, and that implementing BCD in hardware is far simpler than implementing floating point. These sort of things mattered more back when processors had less than a million transistors that ran at a few megahertz.
I'm sure the Wiki article linked to earlier goes into more detail, but I used BCD on IBM mainframe programming (in PL/I). BCD not only guaranteed that you could look at particular areas of a byte to find an individual digit - which is useful sometimes - but also allowed the hardware to apply simple rules to calculate the required precision and scale for e.g. adding or multiplying two numbers together.
As I recall, I was told that on mainframes, support for BCD was implemented in hardware and at that time, was our only option for representing floating point numbers. (We're talking 18 years go here!)