These pots help keep your plants in place. They also help when you want to rearrange your plants. Normally, if you planted directly into the substrate and then tried to remove it there would be a high possibility that you could damage the roots. This could effect the growth of the plant negatively.
Using these pots the chances of this happening is reduces as a lot of the root mass would be within the pot and so to rearrange you would need to just pull out the pot.