One reason you would not apply EA is if you think you can significanly improve your application between the EA and RD deadlines. Examples of this may be if you plan to re-take a standardized tests and the results won’t be in by the EA deadline, if you feel it would help to show another semester of strong grades etc. A second reason you might not apply EA is if your essay is incomplete, your teacher recs aren’t complete etc. (schools can require all documents to be in by the deadline) and you don’t want to rush things.
That said, if your application is in order by the deadline then EA is a great option.