When new leads enter Marketo, they are NOT auto synced to SFDC unless you create a workflow to do so.
If a lead already is synced, then Marketo does automatically update the data for that lead.
So if you have a bunch of new leads from a white paper, not all of them are ready for sales, so some firms keep those leads in Marketo only to nurture them. They will then sync those leads when they reach a score threshold or other MQL status.
I usually prefer to sync it all to SFDC and place nurturing leads in a Nurture Queue, but only with a lot of inbound leads.