Hi,
From the CoolSoft page:
"VirtualMIDISynth is a software MIDI synthesizer implemented as a Windows multimedia user driver, accessible as a standard MIDI Out device."
The CoolSoft synth uses a different technology (driver) than some MIDI synths or sound libraries, and thus is detected by Musician or Composer as a standard MIDI Out device automatically. Not all software synths are designed that way, so those devices have to have a way to connect to other MIDI devices, thus the use of virtual MIDI cables.
For the first few versions our software did not have innate soundfont capability, and so using virtual MIDI cables was one easy way to connect to a number of different synths and sound libraries (and still is for some) and have them show up as usable MIDI playback devices in the Device Setup dialog.
Now that Composer has its own NS Synth where you can select your soundfont (Musician only uses the amazing default
General User soundfont by Chris Collins) you don't need another soundfont synth nor virtual MIDI cables at all for using soundfonts
You may still need them if you want to use some sound libraries, but the NS Synth can use sf2 or sf3 type soundfonts, so it's pretty flexible.
I hope this helps explain the current situation.
ttfn,
Sherry