Well, by the clues I left in my previous post you can find out what they were put in CPUs for, and why their use after the appearance of >256 bytes RAM in computers and non-serial output devices (not to mention FPUs and CPUs with larger words than 8 bits) made them quite obsolete. A computer's base is 2, and it has always been that way, all input data is in base 2, all units are made to calculate it. The reasons for its brief appearance on the scene are temporary practical ones.
I agree the limited uses on large numbers only using certain operations in base 10 is a bit silly. First of all, nobody who needs to operate on really large numbers in base 10 buys a $30 7 MHz CPU to do it, much less an 80 MHz one 10 years later, and certainly not droves of them to compensate for the puny speed. They tacked it on unnecessarily.
Secondly, the instructions are rudimentary for even the simplest math someone would want to do, and they'd do it inefficiently.
Anyway.
I've left some clues for those who want to find out their use way back and don't want the nostalgia spoiled. Would be cool if someone could contribute some code snippet where they're (ab)used to do something useful on Amiga, but...