I'm not trying to be smart, but why does everyone tell me to make a thumb drive installer ie; what does it do differently to what I've done.
A very fair question, hoody.
You could, in fact, go through life with endless new Macs/Operating Systems and never bother with a Thumb Drive Installer. It is certainly not obligatory.
Why then would you bother? Well, it has certain advantages.
The installation of a new Operating System (OS) is necessarily time consuming and depends on a reasonably fast Broadband connection to get the new OS followed by the actual installation time.
And let's suppose that you have more than Mac in the family which you want to upgrade with the same OS.
You can use the USB Installer Drive to put that OS on as many Macs as you wish without downloading the OS for each one.
But it's more than that. When you make the initial download of the new OS and create a USB Installer which is bootable. A USB bootable Installer can, as above, be used to put the new OS on all your Macs. In addition, because the Installer is Bootable, it means that you can Boot into the Mac or Macs, go the Disk Utility (DU) and Erase the Mac's Internal Drive, then Install the new OS on to a "fresh" Internal Drive.
Thereafter, bring over all your data and settings from your backup. This is often called a "clean install" and is helpful if you feel that your Mac was not behaving as it should, perhaps because you had previously downloaded every new OS on top of the previous one.
But strictly speaking to do a proper Clean Install after you've Erased the Internal Drive, many would advise you to move all your data manually from your backup and reinstall all your additional apps from the web. This is a tedious and time consuming process, but it does get rid of all the remnants and scruff that can accumulate over time on your Mac.
But there's more. The Bootable USB Installer can be used to boot into your Mac and erase the Int Drive and reinstall a new, clean version of the same OS - that is the same OS as is on the USB Stick. So even though you aren't upgrading to a new OS, you can do an erase-and-reinstall of the current OS if you have a problem
If my explanation is less clear than you might want, this article from Apple may fill in the gaps:
Use an external drive or secondary volume as a startup disk from which to install macOS.
support.apple.com
Ian