Why is a software update required for my Mac now.

Are you referring to Related Article: If you see "Software update is required to connect to your iOS device" on your iPhone, iPad or iPod touch

If your Mac says a software update is required to connect to iPhone or iPad – Apple Support (UK)

IF so, You may need to update the software on your Mac in order for it to recognise your iOS device. 

Click Install to download and install the software so your Mac can update or sync your iOS device.

It is just a small patch for macOS.

You won't notice that update by the way, it only lasted 5 minutes or so.

The Mac remains macOS version and the iPhone remains iOS version.

If you're experiencing difficulty connecting your iPhone to your Mac, there are a few steps you can take to troubleshoot the issue:

1. Check the USB cable and ports: Ensure that you're using a certified Lightning to USB cable and try connecting it to different USB ports on your Mac. Also, check for any physical damage or debris in the ports that may be preventing a proper connection. Use genuine cable --> Identify counterfeit or uncertified Lightning connector accessories ...
2. Disconnect all other USB devices from your computer, except for your keyboard and mouse. --> Resolve USB-related alerts in iTunes/Finder - Apple Support
3. Restart your iPhone and Mac: Sometimes, restarting your devices can resolve temporary connection issues. Restart both your iPhone and your Mac, and then try connecting them again.
   1. Restart the device. That's it.
   2. Still not solved --> Force restart iPhone - Apple Support
4. Update your software: Make sure that both your iPhone and Mac are running the latest software versions. On your iPhone, go to Settings > General > Software Update, and on your Mac, go to the Apple menu > System Preferences > Software Update.
   1. Update Over-the-Air (OTA) —> Update your iPhone or iPad - Apple Support (IN)
   2. Update using iTunes/Finder —> Update your iPhone, iPad, or iPod touch - Apple Support (IN)
5. Trust the computer: When connecting your iPhone to your Mac for the first time, you may need to trust the computer on your iPhone. Unlock your iPhone and check for any prompts asking you to trust the connected computer. Confirm the trust request on your iPhone. About the “Trust This Computer” alert message on your ...
6. Try a different port, cable, or computer. The issue could be related to your computer, its USB port, or the USB cable connecting it to your device. If possible, try a different computer, USB port, or USB cable. Also, check for any debris in the charging port on the bottom of your iPhone or iPad. --> If your computer doesn't recognize your iPhone, iPad, or iPod ...
7. Reset your iPhone's settings: If the issue persists, you can try resetting your iPhone's settings. Go to Settings > General > Transfer & Reset > Reset > Reset All Settings. This will not erase your data but will reset your settings to their default values. After the reset, attempt to connect your iPhone to your Mac again. Return iPhone settings to their defaults - Apple Support (IN)
8. Update iTunes or Finder (macOS Catalina or later): If you're using a Mac running macOS Mojave or earlier, make sure you have the latest version of iTunes installed. If you're using macOS Catalina or later, iTunes has been replaced with Finder for device management. Ensure that you have the latest macOS updates installed, which should include the latest version of Finder. Update macOS on Mac
9. Check for conflicting software: Sometimes, third-party security software or firewalls can interfere with the connection between your iPhone and Mac. Temporarily disable any security software or firewalls and see if that resolves the issue.

If you are experiencing an issue not listed here, contact support as they can provide specific troubleshooting steps based on your situation.