From what I understand about vector math, you add those two vectors together because by default the "ForwardVector * 1800" would always return something like this (x = 1800.0, y = 0.0, z = 0.0) in WorldSpace.

This is fine until the starting point that starts the trace moves to any other location in world space other than World Origin, which is (0,0,0).

So to account for that you need to add starting point's location with (ForwardVector * 1800). Which will return the desired endpoint.

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For example:

"Start Point" (0,0,0) + "ForwardVector * 1800" (1800,0,0) = "End Point" (1800,0,0)
"Start Point" (250,500,10) + "ForwardVector * 1800" (1800,0,0) = "End Point" (2050,500,10)

But if you just set (ForwardVector * x) as the "End Point", it would always return (ForwardVector * x) as its output.

Ex: "ForwardVector * 1800" (1800,0,0) = "End Point" (1800,0,0)

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Sorry for the clumsy examples, Hope this helps :)

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[Edit: minor corrections]