To answer your question: You need the MyRoute-app in order to navigate.
You create routes in Routeplanner and then (assuming you want to stay within the MyRoute family) you navigate the route via the separate MyRoute-app. If you want to navigate the route you created via, say, a Garmin or TomTom device, the same applies. Create the route in Routeplanner and then send it to your Garmin or TomTom device.
Though Routeplanner and and the MyRoute-app are separate entities (one for route creation, the other for route navigation) they are now interlinked seamlessly.
If you just want to be told how to navigate from A to B, say from Dublin to Cork, you can do it just in the My-Route app. Though for anything more complex or for bespoke routes that you might create yourself, you’ll need, Routeplanner.