Something that has always bothered me is why isn't data set to NULL by the compiler, if nothing else was specified? I can't see any reason NOT to, and surely it must be better than the junk that gets referred to otherwise?
The original reason was "for speed" -- the variables were only going to make sense once you assigned something to them, so there was an effort to not make a pointless initialization to 0 that was just going to be overwritten by the programmer in a few milliseconds with the actual data anyway.
While it isn't exactly expensive, compilers probably don't zero out variables today for the same reason: any compiler that does would look slow compared to other compilers on benchmarks.