Depending on the size and type of plant they can eat up CO2 very quickly, in fact the CO2 around leaves can be used up in as quick as 15min and when the CO2 runs out photosynthesis stops which mean plants stop growing. Also, as was mentioned fans will replicate wind making the plants stronger and able to support more bud which every one wants. You may have enough air movement if your growing in an open room but if you ever want to step up your op or notice your harvest isnt what it should be keep this in mind.