Well, the question "why" is easily answered. You have a bunch of old ATs knocking around and you want to turn them into something useful.
I think TDs use a mix of techs. I have to look it up in-game, but I think they take some techs from ARM and some techs from HARM and I think their guns are from AT... For example, they benefit from both the Medium Tank Armour and Heave Tank Armour advances (one gives them defensiveness, the other gives them improved armour and both reduce speed).
I mean, historically TDs were just cheaper / less complex tanks, so having a TD should make it a lot easier to create a tank than, say, a bunch of horses ^^
Edit: Only problem (if you think it is a problem), this would mean that you could upgrade ATs straight to HARM.