Journey Builder data will always be static - meaning once a Contact enters through your Salesforce Entry Event to the Journey, the attribute values it comes with will always stay the same.
Using Synchronised Data Extensions data in your Journeys means you can always grab the latest attribute values to cross-check or base further decisions.
As Synchronised Data Extensions will auto-refresh with latest data from Sales Cloud in every 15 minutes.
- For an example: you can check if Opportunity Status is still Prospecting after a Contact exits from a 5-day Wait period. If it's still in the old Prospecting status, send a reminder to the Contact or create a Task for the Sales Rep.
Another important thing to note is that attributes that come to your Journey Builder Entry Data Extension may have very long and specific attribute names, depending on the object relationships and attribute paths.
The good thing is that those values will be available right away, when the Contact enters to the Journey with those attributes.
Think of it this way - synchronised data sources is your auto-refreshed source of Sales Cloud data in Marketing Cloud that you can use to personalize content, create smart Journeys, develop multifunctional Cloud Pages and so on. All your Salesforce Data, ready to be cross-checked upon at any time with AMPscript, SQL, SSJS or API's.