Hi Jessie,
I've created a few lead nurture campaigns before that have the Marketo-only form, which I don't sync to SFDC. It's to weed out the bogus leads. So those leads only stay in Marketo, until they are MQL.
But lately, we've had to sync all our lead nurture campaigns in SFDC to track all leads whether good or bad. So when they do fill out the Marketo form, they do automatically sync to SFDC.
I hope that answers your question.