The synth rack works more or less how a rack of hardware synths would. Synths are not depending on tracks at all, same as you can have an available hardware synth and not have it connected to the mixer until you need it. I find it very flexible that I can record midi data in a track pointing to a hardware synth and then change the output to a software synth in the rack to try other sound, edit the patch, then add another synth to the rack to try with the same notes while keeping the other synth with its edited sound, instead of using replace to completely discard the instrument and the edits. Besides, the Synth Rack is where the assignable controls are, like @Astra-Ios points out above.