See this simple sentence:
The dog bit the cat.
It's built of these basic bits:
The dog [subject] bit [verb] the cat [object].
You would never dream of putting a comma after the subject and before the verb:
The dog, bit the cat. [No. You wouldn't do this, would you?]
But when the subject is a longer noun phrase, many people (I catch myself sometimes) will want to add a comma after the subject, just it feels like there should be a pause...