A simple example would be something like predicate pushdown:

    with mycte as (
        select a, count(b) 
        from foo 
        group by a
    )
    select *
    from mycte
    where a = 10;

There is enough information in the statement to plan it as

    with mycte as (
        select a, count(b) 
        from foo 
        where a = 10;
        group by a
    )
    select *
    from mycte;

Before, Postgres would have computed the aggregate for all values of `a`.