The smartphone is a game - changer for the protagonist. He can use it to document his journey, for example, by taking notes or making recordings. This is important for his growth as an adventurer. It also makes him stand out in the new world. People are often curious about this strange device he has, which leads to many interactions. Through these interactions, he learns more about the world and its inhabitants, and his life in this different world is continuously evolving as a result.
The smartphone gives the protagonist access to knowledge. He can look up things he doesn't understand about this new world, like the properties of magic items. So it changes his life by making him more informed.
It serves as a communication device. He can contact people far away, which is crucial in a world where travel can be difficult. This helps him build alliances and get help when he's in trouble. Also, the smartphone has entertainment features. When the protagonist is feeling down or just has some free time, he can use it to relax, just like we do in our world.