Why use start instead of early start when filtering

you can use either 

The Start field is accurate for all activities, while Early Start is only accurate for future activities.

The Start field takes the value of Actual Start when it exists and Early Start when it doesn't.  Early Start never reports an Actual Start date.  Under Retained Logic, Early Start rides the Data Date regardless of progress; under Progress Override or Actual Dates, it is blank if an Actual Start exists.

Makes no difference as long as the schedule is un-progressed or you are only interested in future work.  

The "Start" field will contain the levelled start if you have levelled.  Whereas (as long as you have the correct option ticked in the levelling options) Early Start will continue to contain the forward pass result after levelling. So they can be different.