First, the suggestion to invest in 2 systems is not in order to sell you more or make more money out of you.
On the contrary, having 2 systems that can be “rotated” will save you money in the long run. Like your real hair, hair systems go under a lot of wear and tear- especially while sleeping or if many hours in powerful sun and UV rays. Also, natural day to day usage causes hair to break, hairs to be lost, and over time color to fade a little. Hence, while hair additions and re-colouring are being done, you can use one system freeing up the other to routine maintenance. Also, by rotating systems (switching to a different one every other month) you simply reduce wear and tear on them.