One could look at hardware synthesizers as a collection of component functions integrated into a single unit:
Looking at the Juno-D this way helps us realize that a computer system can provide many of the same functions as a host for a "virtual" (i.e., software) instrument. We could use the computer's CPU and OS to host the instrument, magnetic and optical drives to store data, the QWERTY keyboard and mouse for input and the computer monitor for output. To complete the system, we would need to add a MIDI controller, a MIDI interface, an audio interface and a sound system to the computer.
There is no reason why Roland couldn't put out the Juno-D as a virtual instrument: application software could be developed to have the exact same behaviors as the hardware model, while accessing the same waveform data. There would be no discernable difference in the units (except that the software unit would be cheaper).
Hardware units do have the advantage of being potentially more reliable. The CPU is typically a proprietary RISC (Reduced Instruction Set Chip) designed by the MIDI Manufacturer specifically for that model synthesizer. Since it doesn't have to handle the many kinds of processes that Windows, Linux or Mac operating systems encounter (Internet access, disk access, printers, networking, etc.) it can run more efficiently. Until recently, hardware units had noticeably less latency time between pressing a MIDI key and hearing it sound. Recent increases to computer processing speeds, however, have made this advantage less significant.
On the down side, hardware synthesizers are significantly less expandable than software synths. And they cost more, too. Compare Reason at $400 list with its two drum machines, two samplers, and two very different synthesizers, plus real mixing and processing, to the mid-level capability of the Juno-D at $600, with its limited patch editing capability and preset sound waveform banks.