The primary reason is for correct brake pedal geometry. Back in the day, most GM vehicles were offered with both standard manual brakes as well as optionally with power-assisted brakes. The optimum manual-brake pedal-to-master cylinder push rod ratio (aka 'pedal advantage') is around 6:1, compared to about 4:1 for power-assisted brakes which don’t need as much pedal advantage because they 'boost' the force generated by the average human leg. GM used a common brake pedal with two pushrod holes located about 1 to 1½ Inches apart; the upper hole was for the 6:1 manual brakes and the bottom hole yielded the power-brake 4:1 ratio. When the brake pushrod was installed in the lower hole for use with power brakes, achieving the proper pushrod arc of travel, center of force, and proper alignment with the brake booster/master cylinder assembly piston center-line required a firewall mounting bracket that positions the assembly at a fairly sharp up-angle.