As for why Brera turned up as a cyborg, its quite easy to guess that it was probably the only way to save him due to possible mortal injuries from the incident.
As for Grace, assuming she was a human, it gives you the strength, power, versality of Major Motoko. Very useful for covert and military operations. Of course, I pretty much doubt the Galaxy will allow all of their citizens to have access to equipments and parts Grace uses.
Now, as for people, who knows. Maybe it gives them better and efficient way to communicate. Easier to surf the network, listen to music, watch movies without annoying the guy beside you. It might give people better lifespan since they may avoid certain diseases or organ failure as well as better resistance and durability to their bodies resulting in high mortality rate.