Red wine is made from red grapes
Red grapes are actually purple.
White wine is made from white grapes
White grapes are actually green.
Passive Voice is used to shift the focus in a sentences. Look at these examples:
Grapes and Wine
Grapes are made from wine.
People make wine from grapes.
The first one (passive) does not indicate the agent. The second sentence (active) tells us people made the wine. People usually pick grapes. It is unnecessary information. It is natural to omit the agent (people) and say simply:
Grapes are made from wine
which focuses on the grapes.
Somebody robbed me.
I was robbed.
The first one (passive) focuses on the criminal (somebody) but the second one (passive) focuses on me (the victim).
Somebody stole my wallet.
My wallet was stolen.
The first one (passive) focuses on the criminal (somebody) but the second one (passive) focuses on the object (the thing which was stolen).
Somebody murdered him.
He was murdered.
The workers picked the olives.
The olives were picked.
The first one (active) focuses on the workers who did the work. The second one (passive) focuses on the olives (which were picked).
The grapes were crushed.
The farmer crushed the grapes.
The first one (passive) focuses on the grapes. The second one (active) focuses on the farmer who crushed the grapes.
Passive Voice shifts the focus of the sentence.