In previous releases I've done all networking with bridged adapters. That works fine, but it means you have to be plugged into a network, even if it is just a cable to a switch. Lots of people complained, saying they wanted to install on a laptop that didn't always have a network, hence the change.
In this example, eth0 is *only* for NAT access to the internet, so it is disabled most of the time, except when you need internet access for software updates etc.
Now eth1 and eth2 are used in the same way there were in previous examples, but they now use Host-Only and Internal Network adapters, rather than bridged. Same concept, but slightly different implementation.
If you are installing at home on a machine with a permanent network, you can just use two bridged network adapters and forget about the NAT adapter.