Why do i need to take a bronchodilator before a steroid inhaler?

Depends: If asthma is under control just take your steroid inhaler. You shouldn't need your bronchodilator (rescue inhaler) on a routine basis. On the other hand if you are experiencing symptoms of asthma - coughing, wheezing, shortness of breath, chest pain - a bronchodilator can relieve bronchospasm & airway obstruction permitting the inhaled steroid to reach the more affected breathing tubes.

Different meds: Short acting bronchodilators are used in asthma to relax the smooth muscle of the airway and are usually used for rescue (or premed for exercise induced symptoms). Steroid inhalers are daily medications to reduce inflammation in the lungs and are not rapid onset.