test_alive_hosts_only is a setting for the openvas scanner application to use our new alive host detection. It is a global setting and can only be changed via the config file. This new alive host detection is run before all nasl scripts. Only as alive detected hosts will be tested for vulnerabilities.
The consider alive option is a scanner parameter and therefore can be set for each scan. It just deactivates all alive host detection and runs all vulnerability tests against all specified target hosts. No matter if they are available or not.
So you can imagine that a fast and reliable alive host detection is crucial for the speed of a vulnerability test and that’s what you get with the new but still experimental alive detection.