To get an even burn through the full length of the propellant.
Also, burning from the inside out keeps the propellant tube from
overheating. A crack in the propellant would cause a burn to the
tube wall and rupture before all the propellant is consumed.
View page
Butane is used as a propellant in hairspray.
A propellant helps the hairspray propel out of the can.