The optimizer will allow your phone to automatically switch from 3g to 4g to wifi based. (I told my phone not to use it as I use toggles for to turn on / off the different radios and I don't live in an area where there is wifi everywhere).
You can use the disable feature to prevent unwanted apps to load up in the background or run. I am using this to disable all of the bloatware (NASCAR app, Facebook app, etc.) The apps are still there taking up valuable internal memory space. But they won't clog up your app list and they won't be running in the background.