Maybe some performance optimization hack where you can use a big integer instead of a float for faster math? Skip right past 64bit unix time and give yourself a lot of breathing room?
One week as I was preparing the report, it was clear something had gone haywire. Waste was roughly equal to total capacity. So either we'd lost all of our customers overnight, or there was a bug. Turns out the legacy billing system was using a long to count # of paid bytes and this had overflowed. So it does happen.
If the report runs and says something like "Fatal: Overflow while multiplying customer_space * repl_factor. Consider floating point numbers or a larger integer type" - you'd go "Oops" fix the bug and run it again.
In short, it was for fixed-point digital signal processing. The raw input and output samples were int64_t. We needed to add, subtract, multiply, and accumulate these to do filtering and linear regression with no loss of precision.
Conventional bigintegers weren't an option because the target application doesn't allow heap allocation. So we rolled our own [1] stack-allocated, fixed-width big integer class.
[1] https://github.com/the-aerospace-corporation/satcat5/blob/ma...
Also, 2^64 is only about 10^19, so if you happen to have 20 exabytes that you want to byte-address, you can't do it with 64-bit pointers.
There are also some data formats that are 128 bit, like others mentioned.