Inactive apps will open up some storage on your device with Android 8.1
An inactive app will not take up space on the Dalvik compiler’s cache, thus freeing up some space on your handset. You won't have to do anything on your part to enable this new capability, but because it is coming on Android 8.1 and not Android 8.0, you will have to exhibit some patience.
If you've hate having to worry about microSD cards, but still want need a lot of storage for all of those apps you install on your phone (many of which you've never opened even once), it appears that this is the best solution for now.
source: Google via XDA