With iPhone charging you more for storage when you buy a phone in some cases if I can not afford to buy a phone with 128GB or storage I will make use of an SD card so that I have more space on my iPhone without paying the hefty price to have it already when I purchase the phone. It's not very often that I need to use an SD card now though as I do not have many apps on my phone with me doing the majority of things on my PC.
I don't usually save big files on my iPhone. It's only few songs that I have downloaded on my smartphone. It's more of my pictures and few applications that are on my iPhone. I have no need for SD cards.