Am I just dumb or does anyone else not understand this math in guns?

Hello. I have this code:

    local mouse = game.Players.LocalPlayer:GetMouse()

    mouse.Button1Down:Connect(function()
        local Bullet = Instance.new("Part")
        Bullet.Parent = game.Workspace
        Bullet.CanCollide = false
        Bullet.CFrame = game.Players.LocalPlayer.Character.HumanoidRootPart.CFrame * CFrame.Angles(0,math.rad(90),0)
        local Direction = (mouse.Hit.Position - game.Players.LocalPlayer.Character.HumanoidRootPart.Position).Unit
        Bullet.AssemblyLinearVelocity = Direction*100
    end)

The line I am intersted in is:

local Direction = (mouse.Hit.Position - game.Players.LocalPlayer.Character.HumanoidRootPart.Position).Unit

That line and this give the same result:

local Direction = (mouse.Hit.Position).Unit

I don't understand why we need to use the first one? Can anyone explain the math?