Why are stale air pockets important to avoid?

Stale air includes bio-effluents (body odours and exhaled breath) and airborne pollutants (such as smells from cleaning products and furniture) amongst others. Air conditioning plays an important role in air distribution within a building. Ventilation is also a very important way of diluting any airborne pathogen. Details are given in CIBSE COVID-19 Ventilation guidance version 3: https://www.cibse.org/coronavirus-covid-19/emerging-from-lockdown