I notice the two people who have answered only mentioned calls and text.
But of course with a smartphone you can listen to music, watch TV, watch a film and so on.
You can also look at the internet, read a book, do social media, and so much more.
With a smartphone you have access to the whole of the internet wherever you are. to find a map of an area, find a local restaurant or pub, find a local museum or art gallery and so on.
I am not a heavy smartphone user, but the other day I was in my town centre on a Saturday afternoon and wanted to catch up with the football scores.
I used my smartphone to watch the BBC TV coverage live for 15 minutes while I got the train home.
THAT is why smartphones are useful, not for the occasional call or text.