The eVOA is simply the online version of the same visa you can also buy as a Visa on Arrival (VOA) directly at the airport in Indonesia — legally and in terms of validity (30 days, extendable once by 30 days), both are identical. The difference is in the process: with the eVOA, your visa is already secured before departure, so there's no need to queue at the airport.
The VOA at the airport is only issued after landing, at a counter on site, which can mean long waiting times especially during peak season. If you want to save time and have more certainty, the eVOA is usually the better choice.