While emoticons and emoji are often thought of interchangeably, and there are even certain e-mail clients that will convert typed emoticons to their emoji equivalent, there are still many that do not.
Based on the description of the add-on I would imagine it is still entirely useful in any context where an e-mail message is in plain text and contains emoticons.
Brian - Windows 10 Pro, 64-Bit, Version 1809, Build 17763
The color of truth is grey.
~ André Gide