Let's say that 3 GB of your internal storage is used by the system, leaving you with 5 GB of usable space, and your SD card is empty.
Currently, you have 5 GB usable for apps (internal storage) and 21 GB usable for media (both the internal storage and the SD card). Note that the media storage will still be divided between the two storage locations, which makes it hard to manage. Installing apps/app data will reduce the available space for internal media, and vice versa.
Under adoptable storage, you will have 21 GB for apps (internal storage + SD card) and 16 GB usable for media (SD card). Note that each app is either fully on the internal storage, or fully on the adopted SD card. Installing/moving apps and their data to the SD card will reduce the available space for media on the SD card, and vice versa.
Essentially, your choice is: Allow media to also be on the internal storage, or allow apps to also be on the SD card. (Plus, in the latter case the SD card will no longer be accessible when removed from the device unless decrypted.) You cannot have both; both storage locations remain separate and are not combined into a single logical partition. The only difference is what is "allowed" in each spot and how it can be accessed.
Please refer to this reddit post for more background on how this works.