Note: This page is taken from n.wikia.com.
(As written in the N in-game help)
Make sure you've got a landing planned or it may end up being a one-way trip.When the ninja comes into contact with a launchpad, it launches in a set direction (depending on which way the launchpad is facing). This can be used to reach otherwise inaccessible areas, speed the player up, or kill them when they smash into objects too quickly from the launchpad's added burst of speed.
Another useful feature of launchpads is that the player may bolt into one at any speed and they will not be killed, as they would by smashing hard into other terrain objects. Many levels take advantage of this, by forcing the player to aim at hard-to-reach launchpads when falling. Also, some levels may use launchpads as a guide of some sort to help the player get to where they're going.
There is one known trick for the launchpad, which allows to perform a jump much higher than usual.
Another well known use for the lauch pads is using it to teleport in some cases.