Making a tank chassis a tank destroyer gives it +hard attack and piercing, at the cost of basically 0 breakthrough, and also has the hidden bonus of making it cost half as much since tank destroyer battalions require half the vehicles compared to tank battalions. Effectively this turns them into something like a more expensive anti-tank gun that provides armor and a bit of hardness.
No breakthrough means I don't see the point of combining them with tanks (which are all about breakthrough and going on the offensive) and more likely to put them in divisions that will be on the defensive, usually infantry. A speed 4ish tank destroyer can be pretty cost effective in this role and will often provide the division enough armor most AI divisions can't penetrate it, all while being a lot cheaper than you'd think. Especially if you manage to get the super-heavies tech; a super heavy tank destroyer battalion is only 7 vehicles and thus often shockingly cheap, and on the defense the massive terrain penalties for a super-heavy don't matter.
If you're expecting to be fighting mostly soft targets you could use self propelled artillery in a similar role, which has more soft attack but the battalions use more vehicles and are thus more expensive. So I prefer to use tank destroyers and that way my infantry also get the piercing and hard attack they need to deal with attacking tanks. Plus some of the "tank destroyer" guns also have a decent amount of soft attack as well so still perform nearly as well against soft targets as artillery.
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