Why should i add 1 ressource to that card?What is it used for?I understand to gain 1 plant but the ressource.
It will depend on which card you have played. If you play a green card with the tag you will obviously collect a plant. If you play a blue card that collects a resource then you can collect one of those resources for whatever effect it has.
Example you play Tardigrades which has a microbe tag and so triggers Viral Enhancers you can add a microbe to Tardigrades.