Does using Projection Expressinos in querying DynamoDB reduce the read capacity unit comsumption

Question

Have been going through the DynamoDB documentation, but couldn't verify the following.

1. What is the main purpose of ProjectionExpression in DynamoDB?

2. Does it reduce the read capacity unit consumption?

3. If not - Dynamodb charges the same amount irrespective of if you read the entire item or just a few attributes?

Any help is appreciated! Thanks!