Formatting as Internal Storage means the phone regards the card as part of its internal storage -- so it can use the card to install apps to. On the surface, this sounds great, but there are a lot of pitfalls. SD cards are inherently much less reliable than internal storage, so if the card fails, that can cause loss of apps and app data, and cause the phone to crash. Formatting the card as Internal Storage also encrypts the card to work only with that phone, so if you were to take the card out and put it in another device, you wouldn't be able to read anything on the card. SD card read/write speeds will be slower than on internal storage, so performance can take a noticeable hit. And the increased read/write operations on the card also accelerate its demise (since cards have a finite read/write lifespan.
So I agree with mustang7757, just use it as Portable Storage, which is best for storage of media files (photos, videos, music) and documents. And don't rely on the card as the sole storage for important data -- make sure you back it up on a regular basis.