The wellness center allows you to recover your health by the treatment action you choose. It takes time for your health to raise up after starting a treatment. You can always interrupt a treatment if you no longer want it as you may want to do another action in that time. Getting your health up allow you to do more actions in the game as some actions use up health from your health bar. The max points of health bar for normal citizens are 100, while for PRO membership citizens are 200.