I am a beginner working with the LibGDX library. Recently I have stumbled upon
First of all LibGDX is a framework, not a library... It's more like a set of libraries that you could use for game development and other stuff. When you multiply a 2D vector by a scalar you just multiply it's "x" value and it's "y" value for that scalar. For example if <4,2> is a vector and you multiply it for the scalar 3. The resulting vector would be <12, 6>. Even when the method says:
That does not mean you only can multiply it by the render delta time. You can multiply any vector by any value you want. And for the last question. Why would you use it? Well, it really depends on the exact problem you are trying to fix or the game you are developing. One posible reason I can think of is if your "velocity vector" (Because Vector2 can be used for velocity, position, etc) is suffering a "scalar acceleration" so every frame you are multiplying this velocity for a scalar and the object will be faster every time. Here's a person who needed this method for something LibGDX Multiply Vector2 with float value. So, you see, if you need it, you'll know it.
Hope this helps.