Some software synthesizers, Koblo Vibra 9000 for example, actually compete with the best analog synths of yore. Old analog faves like the SCI Prophet-5 are slowly being condemned to the expendable list as well. There's not much left that you can't do with your computer. Plug the soundcard into a pair of active monitors, and you're ready to go. Cabling is kept to a minimum, reducing clutter and potential noise problems.
Computers are upgradeable, another key advantage.
Add new software, a better soundcard, a bigger hard disk, or a faster processor, and your machine gets a new lease on life. Try that with your old multitrack tape machine. Some plug-ins have established new trends. Line 6 AmpFarm kicked off software amp simulation. With today's accelerated production schedules, that convenience is a big plus.