As stated before, synergy is a nice alternative to spending money, i mean, what happens when you can't externally install the program(say on a freshly made pc). I think synergy could double for parents who want to keep 'tabs' on there kids browsing by the looks of it.
Regardless, synergy or a kvm switch are two nice pieces of equipment/software that can be used to do the basic, which is eleminate the need for a completely new/spar pc and monitor. I think the better advantage to the KVM switch is the ease of use, plug in and your done.
I wouldn't suggest using synergy on a customers pc(if your doing at home work) that's when a KVM switch comes in handy.