You can take a signal and send it to two different virtual instruments without having to duplicate the instrument the original signal came from.
So if you had a matrix sending signals to a Malstrom and you want the same pattern to play in a Subtractor, you can use a splitter to send the signal from the same matrix to the Subtractor without duplicating it.
This is the most basic use for it but you can do a lot more with it when you get a little more comfortable with the idea of what CV Note outputs are and what they are for. On the propellerhead website. There are 7 tutorials on there that help you understand what you can do. They give you a downloadable reason file so you can see what they are doing.