It's possible to stack signs on top of each other. You can do this to create the illusion of a really big sign that is actually comprised of smaller pieces. It's perfect if you have a long message to write or something like that.

The best use for this mechanic is when you're making an adventure map. You can stack signs to create a literal wall of text full of instructions and details about the adventure. Stacking signs is a small detail that could easily be missed.