benefits vs drawbacks to storing apps on sd card

So I apologize in advance if this seems like an elementary question but so it goes. What are any benefits and/or drawbacks to storing apps externally(SD card) vs internally(phone memory)? Of course other then the obvious to free up space on ones internal storage are there any other benefits? drawbacks? Such as if apps are stored on the SD card can they be transferred to another phone? The "apps to SD" app allows one to also transfer externally stored apps to the internal phone memory...when would one want to do this? What would be a benefit? Thanks for all who help this newbie