What is they advantage of a iMessage?

iMessage is free between iOS devices and can share Photos and Videos in its original format with other iPhone users with many other data easily over the internet as compared to SMS/MMS

iMessage uses Internet protocol

SMS uses Voice Protocol (Does not need Internet)

It uses data rather than SMS charges. If you have unlimited texts then there is no financial benefit when sending words..

Sending images and videos via iMessage (ie to other iPhone users) is usually a lot less expensive that MMS which can be charged in real cash in some countries.