The StuG was indeed designed as an infantry tank, yet it ended up as some kind of turretless hybrid.
After being upgraded to 80mm frontal armor @ 10-20° from model F-8 onwards, it proved to be a cheap counter against the T-34/76, which had quite some trouble to penetrate the reinforced front at high to medium distance, unlike against the Panzer IV series with their lower 50mm turret armor. And thank to their 75mm/L43, the StuGs had better penetration than the Panzer III series, which also had adequate turret and upper hull armor, yet lacked pentration power with their weaker 50mm/L42.
After being upgraded to 80mm frontal armor @ 10-20° from model F-8 onwards, it proved to be a cheap counter against the T-34/76, which had quite some trouble to penetrate the reinforced front at high to medium distance, unlike against the Panzer IV series with their lower 50mm turret armor. And thank to their 75mm/L43, the StuGs had better penetration than the Panzer III series, which also had adequate turret and upper hull armor, yet lacked pentration power with their weaker 50mm/L42.