The Rapist, therapist ... both would pass a spell checker, but imagine the use of either in the wrong context.
Spell checking is not proof reading
Let's be honest, in any document the wrong word might be meaningless or subsequently extremely expensive if it was part of a contract. We proof read documents of any length.
We spot mistakes like 'their were 3 of them looking at there work'
'Christopher Columbus circumcised the world'
The team were ready.
During a busy stressful week it is foolish to ask your colleagues to proof read - they will make unfound errors - ask any printer about costly mistakes.
The Rapist, therapist ... both would pass a spell checker, but imagine the use of either in the wrong context.