My reasons are as follows:
* SD cards are a *LOT* cheaper than additional built in memory.
* SD cards can be removed and connected to my PC for file synching with whatever software I choose.
* SD cards can be moved between handsets (my current SD card is from a previous phone originally, I swapped it over when I bought a newer phone and BAM - 60GB of music instantly available).
* SD cards survive if the phone dies - my LG G4 inherited the infamous boot-loop fault. Everything on internal storage was toast, SD card was fine.
* SD cards work when data isn't available, ie camping, poor signal areas, and especially when flying.
* SD cards also allow for a cheaper contract with less data required than if you try to stream everything from cloud services.
I could go on. Internal storage is inflexible and troublesome, streaming from the cloud is costly and not always available. SD cards are cheap, flexible and always available. What's not to like?