Yes, you're right, actually what we need now is just the single Expression. 
But I put the projection who contains multiple expressions/fields, because:
1) We may have the case which need to computing multiple fields for a single input row. (Of cause that can be replaced by multiple expressions with different row feeding.)
2) In the future expression evaluation optimization, I am thinking to CSE(Common Sub expression Elimination), the Projection may hide the details in each expression computing.