It's basically one extra episode in the game. It isn't necessary of course, but it will give you some more background as to what happens to Alan once he reached the Dark Place. It's small though and subtle. If you go for The Signal, I'd recommend going for The Writer to.
The DLCs try to have the levels play out differently than the rest of the game too. The Writer comes off as more platforming and The Signal changes up a few scenarios where you'll fight and flee differently.