The most useful feature for me was that I could see how many people were open with my emails at any given time - so if it wasn't getting read or opened then you knew right away! It's also great when trying out different layouts/ designs as well which can be really helpful in terms of finding what works best within your target audience; however this may take some trial & error before seeing results (as there are several factors affecting conversions). Although not very obvious from lookingโฆ
Pros
Flexibility, allowing to personalize email templates.
Automation features including sending scheduled newsletters etc., making automation much easier compared working manually without software assistance