For finance probably but it's mostly useful for statistics from what I understand.

I know fortran and Cobol code that banks use and is used for science (tm) often defines numbers this large for certain operations.

One such i can think of is a map reduce of large datasets. Let's say you want to find as a result a huge sum. 2^128 is a bit bigger then 2^64 and that difference may be big enough to provide the computation needed for getting to Mars rather then getting to the moon on 2^8 machines.