It's annoying and unnecessary for most
Here’s a simple tip for the day: stop iTunes from opening up automatically whenever you connect your iPhone, iPad or iPod Touch! It really annoys me because I don’t even really use iTunes anymore, so there is really no need for me to open it. If I need to perform a local backup of my iPhone, which I do every couple of months, I can open iTunes myself and do it.
Go ahead and open iTunes and then connect your Apple device via USB. You should see it pop up in the left hand sidebar. If you’re using iTunes 12, there no longer is a sidebar, so instead you will see the device show up at the top right next to the iTunes Store button.
Now go to the Summary page and scroll down to the bottom where it says Options.
Uncheck the Automatically sync when this iPhone is connected box and you’ll be good to go! Now whenever you connect your phone to your computer, it will not open iTunes.
If you still want to sync your iPhone to your computer, you can check the Sync with this iPhone over Wi-Fi box. Now the iPhone will sync automatically when the device is charging, iTunes is open on the computer and both are connected to the same WiFi network.
With iTunes syncing, you can sync applications, audio content, books, movies/shows, photos, documents and ringtones. For other stuff like calendars, contacts, notes and bookmarks you can sync all that using iCloud. I personally never sync my phone with iTunes anymore, but if you still find a need for it, this is one option without having to have iTunes open all the time whenever you connect your phone.