You would use [field1] as the control source if [field1] was the name of field in the underlying table/query.
=[field1] works exactly the same in this case.
You would use = as the first character of the control source if you were refering to something that was not a field in the underlying table/query or if you were modifying that field. ex: = [field1] & " Some Text" . (Note: if you modify [field1] you cannot use field1 as the name of your text box else you will get an error.