One of the most common reasons for disabling the auto-sync feature in iTunes is to make sure that any songs that have been accidentally deleted from your main iTunes library won't also disappear from your iPhone.
It may be easy to get your iTunes purchases (music, videos, apps, etc.) back from iCloud, but what about all that content that hasn't come from the iTunes Store? Unless you have a backup somewhere (like iTunes Match or an external hard drive), songs that are accidentally deleted may be unrecoverable if iTunes has also deleted it from your iPhone.
The reason for this is because syncing songs and other files via iTunes is a one-way process. When you delete content in your iTunes library, this change is also mirrored in your iPhone, sometimes resulting in the accidental loss of non-iTunes sourced material.
With macOS Catalina (10.15), Apple renamed iTunes to "Music." This guide includes instructions for both and uses the terms interchangeably.
How to Disable Automatic Syncing in iTunes
Turning off the auto-sync feature in iTunes/Music should take just a few minutes. Here's how to do it:
Disabling automatic syncing means automatic backups will not take place. Part of the iTunes syncing process involves backing up important data on your iPhone, so you'll need to do this manually once you've disabled this option.
Open iTunes/Music and open the Preferences window.On a Mac computer, select the iTunes menu > Preferences... On a Windows computer, select the Edit menu > Preferences...
Select the General tab, then disable the Sync Library checkbox if it is not already.
With older versions of iTunes/Music, you may to select the Devices tab, then enable: Prevent iPods, iPhones, and iPads from syncing automatically.
Select OK to save and exit.
iTunes can now only perform file synchronization to your iPhone when clicking the Sync button. However, before connecting the iPhone to your computer it's a good idea to exit iTunes and then re-run it. This will make sure the settings you changed are reloaded and actively working.
Manually Manage iTunes Media
Now that you've disabled automatic synchronization between iTunes and your iPhone, there's another option you can use to switch iTunes into manual mode. That way, you can selectively choose what music and videos should sync to your iPhone.
Open iTunes/Music and connect the iPhone via USB. After a few moments, your device should be recognized in iTunes.
Select the iPhone on the left pane of iTunes, under Devices, to see a summary detailing library contents, backup settings, options, and other information. If you don't see this screen, select the small phone icon at the top of iTunes, right below the menu.
Scroll down the summary screen until you see the Options section. Select the checkbox next to Manually manage music and videos to enable it.
Select Apply to save the settings and switch to this manual mode.
Rather than all songs and videos being automatically synced to the iPhone, you will now have direct control over what songs and videos end up on your device. Here's how to manually move songs over to your iPhone:
Select Library at the top of iTunes.
Drag and drop media from the main screen on the right to your iPhone's icon in the left pane.
You can select multiple songs or videos on a PC with the Ctrl key, or on Macs with the Command key. Do this for as many as you want to highlight at once, and then drag one of the selected items over to the iPhone to drag all of them simultaneously.