Because I am quite excited about it myself and I think this is something that a lot of children may not have opportunity to do otherwise. Model boats are relatively simple to build but there are a lot of parameters you can vary and see how that affects the boat. The good thing it teaches is the trade offs. You may make the boat faster but less stable, reduce it’s range or mobility. That’s where the engineer has to decide what is more important and come up with an optimal solution.
And I think boats are great fun, hence it is a good way to get children excited about engineering work.