Yet there are those who do, because they want to store their massive music database on the device, without being limited to cloud and they need to change their battery so they can listen to that music they have store on their phone all day (when, say, on travel).
Different user types, some need the "un-necessities".
For me the SD was somewhat necessary, because my S4 mini has only 8 GB on board at the first place. Second reason with my 32GB SD I have all the room open to add much more data/music and make virtually unlimited amount of photos while at that and can upgrade to 64GB if necessary. Third reason is that I have my photos and music stored there, I can flash and wipe my phone and not move stuff, or in case it bricks or breaks, chances that I can retrieve the data from the phone without problems are rather high.
SD rocks in may ways. I use my PC in similar ways, got my data and games stored on separate drives, so if I need to format the OS drive, everything there can/needs to be reinstalled.