As a developer, it can be annoying to find the position of the mouse relative to a ui element (and vice versa). This is useful for running drag and hover effects, and it can be very complex to implement when you’re working with something like a BillboardGui or a SurfaceGui.
Those are different from what OP’s looking for. The methods in the Camera object deal with transforming points to and from the viewport (3D view) onto the screen (2D view) as well as constructing rays for raycasting. OP wants a 2D transform for relative position checking (like CFrame Object/World space but for 2D UI).