It really is best for storage for your videos and music (and other things). I do not recommend moving or installing apps onto the microSD card. It is not as fast as internal memory, although it is fast enough to record videos and photos, as well as watch videos or listen to music. You can encrypt the card like you can the internal memory, just be aware that if you do, you must decrypt the card before using it on another phone.
I have a Samsung EVO+ 256GB microSD card in my S8+ and I have my entire music collection on it. It is also the default for video recording and taking photos. I haven't had any problems with music or video playback. Even with my entire music collection, I still have about 70-80GB of free space.
The S8/S8+ is capable of using microSD cards up to 2TB. Currently the largest available is a 400GB card made by Sandisk.