The main reason I've seen this done is to make the changes from Marketo into SFDC as real-time as possible.
As you may know, the Marketo/SFDC integration keeps both systems updated with the latest changes by checking in about every 5 minutes, and so sometimes a change made in Marketo will not be reflected in SFDC until about 5 minutes after it's updated in Marketo.
Adding the "Sync to SFDC" flow step forces Marketo to update SFDC instantly rather than waiting for the next scheduled system sync.
In my experience it's rarely necessary to do have anything done "in real time" and when I see this rule scattered throughout Marketo I often see it as a sign of insecurity by admin in the Marketo/SFDC integration itself. At some point the admin probably made a data update in Marketo, went to look to the change in Salesforce, didn't see it, didn't wait long enough to see it, and then tried the whole thing again with the "Sync to SFDC" in the flow - and it worked! And so decided that "Just to be safe, I'm going to add a Sync to SFDC flow step to every campaign."
Having said that - I don't know anything about the specific requirements that your past admin might have had at the time- I'm just speculating based on what I've seen other companies do.