Because if there is too much light and the plants are not enough to take the nutrients then algae will do it and you will end up with all the plants and other stuff covered in algae. Still, you don't know if you will need co2 for your light intensity.
In other words...
If you have nutrients that can't be used by plants (lack of co2 or too much light), then algae will show up to make up for the amount of photosynthesis that needs to happen in relation with the amount of light/available nutrients.
This is why I advised you to start heavily planted. It even helps the cycle as the plants are covered in biofilm that would inoculate the tank.
Having a planted tank is always working with this balance between light/co2, plans/algae...
Do you know why could you need co2?
A heavily planted shrimp tank is possible!