A Panther will protect you against pretty much every tank in the game, a Sturmtiger cannot stop a WC 51. That's hyperbole, but a Panther is never a bad choice, while a bad call on the Sturmtiger will ruin your game.
no one is saying you should go a sturm every game. there are situations that might arise where a sturm fits in perfectly however. its a very specialist and niche unit that doesnt see much use.
sure the panther is a good tank: high armor, high hp, good speed, but it doesnt particularly excel in breaking up defensive lines. it doesnt delete whole weapons every minute, rendering them unusable.
its easy to say, oh you can go stuka, or oh just go KT. but what if you go T1 > T3? you would have to backtech to T2 to have access to the stuka, which fires less frequently and sometimes doesnt destroy the weapon. and honestly you have to wait so long to have enough fuel for the KT, sometimes you dont have the luxury of time and want to press your advantage as soon as possible.
ultimately though its just another tool in your arsenal. its never gonna be the most commonly used tool, but it does have its moments.