So, the above poster gave a very good answer.
I just wanted to add a few things.
When you install windows (a fresh install) it frequently installs device drivers for all the things on your motherboard. How your motherboard handles internet, how it handles sound, how it handles memory interface, how it handles hard drives, how it handles the interface that talks to your graphics card are all installed.
If you just transfer the hard drive and boot windows, it will try to use those device drivers to boot to windows... but your new motherboard doesn't use the same devices. So sometimes windows will catch it and install new 'chipset' drivers. Sometimes that works but sometimes it doesn't. Sometimes because it is trying to run the wrong devices according to how it was installed windows will develop problems. Those problems might include becoming VERY VERY slow as it waits for the devices it expects to respond. Sometimes it will install a driver on top of the old driver instead of deleting the old driver and things won't work... so you might have to manually install a sound driver, or half of your USB ports might be inactive.
When you do a fresh install of windows it doesn't have those potential problems.
Remember also that if you install a fresh install of windows you will need to go to the manufacturer's webpage and download all the drivers for your motherboard and install them.
This is a hassle, but it makes sure that your computer will work properly.
So, when I do it I backup all the files I want to save (an external USB hard drive is a good device to use for this).
I then transfer the things I want to reuse into the new computer... this includes the hard drive that has the old windows on it.
I have previously downloaded the newest version of windows 10, and put it on a thumb drive.
I boot the new motherboard from the thumb drive and it starts the installation process. I choose a fresh install, rewriting the hard drive.
Then I change the boot order to boot the newly installed hard drive that holds windows, go on the internet to the manufacturer's webpage and download drivers for my new motherboard and drivers for everything else I have in the new computer (graphics card, printer, whatever).
After I install all of those I force a windows update and install all the updates.
Then I force a windows update and install all the updates.
Then I force a windows update and install all the updates.
Then I hope that all the updates are done, but just in case I force a windows update an check and see if I need to install more updates.
THEN I'm happy and I start installing steam and various games.
Hope this helps!