Those that have purchased EBCI will be checked in, by SW, 36 hrs prior to their flight departure vs the passenger checking themselves in at the 24 hr window. This only gets you checked in 12 hrs prior to those that haven't paid for EBCI.
You will get a lower boarding slot than those that didn't buy EBCI.
Will it get you a better seat? Depends on what you consider a better seat. There could be 18 people boarding before you, or there could be 50 people boarding before you....even if buying EBCI. Depends on when you bought EBCI and how much you paid for your fare. And even then, if you bought EBCI 4 months prior to your flight, if someone that paid for EBCI bought it before you, or paid full fare, and then cancels, someone coming along down the road could snag that particular place in line...so to speak.
But EBCI gets you checked in at the 36 hr mark, which usually gets you a decent boarding spot.