Some people say MicroSD cards are not needed anymore with the high internal storage capacities available nowadays. But that is not true. MicroSD is about control, not only storage.
MicroSD cards let you quickly move large amounts of data between devices.
MicroSD cards let you instantly free up space by swapping it with a new card.
MicroSD cards let you access your data if you break your phone or if some update has a bug that makes your phone unuseable. You have the peace of mind that if you break your phone, you can easily access your data.
MicroSD cards are pay-once own-forever. Cloud storage requires perpetual payment and cloud storage providers may be snooping in your photos.
Many sports cameras have MicroSD card slots. If your phone has one, you can watch your sports camera video on your phone without needing bulky adapters.
What if you need more storage than you thought you would need? What if you bought a 64 GB phone but suddenly realized you need more? You don't have to replace your entire phone.
Not many phones have MicroSD card slots nowadays, probably because phone manufacturers fear that people buy cheap off-brand MicroSD cards and then blame the phone for the slow performance. This can be solved with a warning message that tells the user to buy a higher performance card or expect performance losses.
There is no excuse not to fit this tiny slot in a phone. It doesn't take much space but adds lots of usefulness.