In general, Office Add-ins provides advantages over add-ins built using VBA, COM, or VSTO. I can name few of them,
Office Add-ins run in Office for Windows, Mac, iOS, and Office Online. So your solution can run in Office across multiple platforms, including Office for Windows, Office Online, Office for the Mac, and Office for the iPad.
Single sign-on (SSO):
Office Add-ins integrate easily with users' Office 365 accounts.
Centralized deployment and distribution:
Admins can deploy Office Add-ins centrally across an organization.
Easy access via AppSource:
You can make your solution available to a broad audience by submitting it to AppSource.
Based on standard web technology:
You can use any library you like to build Office Add-ins
Hope this helps.