My phone is on it’s 4th battery. If this one dies I can buy another one for 20$ and have them swapped in 20 seconds. My sd slot allows me to carry my entire music and game rom collection as well as a decent selection of movies. If I wanted it would be trivial to load the rest on SD cards and swap them out. The headphone jack allows me to use Wired headphones that dont cost 100+ dollars and don’t have batteries that need to be charged and wear out. It also lets me plug my phone into my cars AUX jack and play the aforementioned music collection.