Plants need CO2 to grow, and the air around us holds about 400ppm. Without adequate exchange, levels fall — growth slows and stalls near 200ppm. Ventilation also removes heat produced by lighting; the sweet spot in a grow room is 24–28°C.
Aim to replace the full volume of air in your space every 20 minutes with a properly sized exhaust fan. Mount exhaust high where hot air pools, and place intakes low on the opposite side to draw in cool, fresh air.
An oscillating fan in front of the intakes spreads fresh air evenly and prevents 'air channelling', where some plants get more CO2 than others. And make sure exhausted air can't loop straight back into the room — intake and exhaust should draw from and dump to different areas.