Q: Updating my iPhone deletes everything if I don't backup it?

As described in this article - Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support - there is always a chance for data loss and it is best to back up your device prior to an update of iOS. However, a normal update should not remove any data form your device.

Your backup is your only insurance in the event something goes awry or otherwise not as intended.

Backing up your data only takes a few minutes compared to the time it would take to restore your iPhone in the event there's a problem with the update.