If the software needs to interface directly with legacy systems, drivers or libraries, then you may need to supply a 32bit32-bit version, since AFAIK the OS generally (definitely Windows and Linux AFAIK) doesn't allow mixing of 64bit64-bit and 32bit32-bit code within a process.
For example, if your software needs to access specialty hardware, it's not uncommon for customers to operate older models for which only 32bit32-bit drivers are available.