Sometimes you need to insert a comma for no other reason than to avoid the misreading of a sentence. Consider these two sentences:
In the place where the willow grew the river was broad and slow.
Once he had been allowed to attend the exorcism of a boy in rural Nigeria.
These sentences could be easily misread. In the first sentence the reader may think you’re saying that the willow tree grew the river (that would be something to see) unless you place a comma after the word grew.
In the second sentence, if you don’t put a comma after the first word, Once, the reader will take that word as a conjunction that means “at the moment when” or “as soon as.” If you use this word as an adverb, it means “at some indefinite time in the past.” To communicate that meaning you have to place a comma after Once.
As we would expect Freud’s self-evaluation would hardly be agreed upon by everyone.
To clarify the meaning of this sentence, you should place a comma after the word expect.
When I woke up Annette was looking down at me.
The first-person narrator did not wake up Annette. He woke up and saw her looking down at him. To make that clear, put a comma after the word up.
When we had finished eating Robert and I left the room.
Unless you’re writing about cannibals (“Pass the salt, please”), nobody ate Robert. Lucky for him. Put a comma after the word eating.