Essentially, the TV app serves as a home screen for all the content available via Hulu, HBO NOW, STARZ, SHOWTIME, NBC, FOX, and any other streaming service or network you've got a subscription to (whether as a one-off or via your cable provider), plus whatever TV shows or movies you've previously purchased on iTunes. Rather than toggle through all the different apps to find a particular show or movie, it aggregates them all in one place so you can universally search and hop directly to what you want to watch within the app it's offered. The trouble is, it doesn't support the biggest streaming provider of them all: Netflix. And that sucks big time.
Considering an enormous swath of streamable content is absent from a database intended to help you find what you want to watch, TV isn't very convenient. And frankly, with a name like "TV," trying to explain what it even is and what it does to anyone but diehard Apple users is a recipe for incredible confusion.