It's about what you put in them. Some people want to put huge collections of lossless audio files at 10-20mb each file, hence they need a bigger SD card. I personally used to carry around more than 30GB of music files around before I started relying on Spotify. I know of others with music collections of more than 100GB
Some others want movies at 1080p, and each of those can be up to 20GB in size. Sometimes people put entire series in their devices too.
Nowadays I store a lot of books. 64GB is actually quite the bottom I'm looking at. So 128GB would be nicer.
128GB isn't necessarily for me. But for others it may be.
Nowadays, it's not really a question of do you need the extra space. You want as much space as you can afford.