A 64-bit address bus allows the processor to address 18 million gigabytes as opposed to the mere 4 gigabytes allowed with 32 bits. There are in 1996 already hard disks which can hold over 4GB. Floating point calculations can also be more accurate.
A 64-bit OS is needed as well to take advantage of the CPU. In 1986 there are only a few 64-bit operating systems, including OS/400, DigitalUnix, Solaris (partialy). A 32-bit OS can run on a 64-bit CPU.