Why is it important for plants that carbon dioxide uptake at day should be greater then the release of carbon dioxide during night?
The intake of Carbon Dioxide is used to make sugar the release of Carbon Dioxide during the night and day uses the energy stored in the sugar.
If the amount of Carbon Dioxide released in the process of respiration was greater than the amount of Carbon Dioxide used in the process of photosynthesis the plant would be "starving" and eventually die.
Plants can store excess sugar during the day and summer months to survive during the night and winters when photosynthesis can not occur. This excess sugar is stored in roots, and sap ( see maple sugar production)
It is important to realize that while photosynthesis the light reaction occurs only during the day, respiration occurs in the plants both day and night. So during the day the plant is both taking in Carbon Dioxide to make sugar and burning the sugar to release the energy needed to live.
Photosynthesis taking in Carbon Dioxide and Respiration burning sugar and releasing Carbon Dioxide are mirrored reactions.