Why do I have to update my phone?

You should update your iPhone because the update may include some improvements on how the software will run on your phone, security changes, bugs and even new features

https://support.apple.com/en-ph/guide/iphone/iph3e504502/ios

updating your phone is really important. it can strengthen security on your phone, fix bugs, improve battery life, and add many more things too. if you want to be safe about it, you can back up your phone with a computer before updating. hope this helps!