[Hobbit: battle of five armies] why would goblins purposefully blind a Troll meant for battle, cut off his arms and legs and swap them for metal weapons, how does that make him more stable or better in battle?

https://www.warhammer-community.com/wp-content/uploads/2017/05/Troll2.jpg

https://i1.wp.com/elanillounico.com/wp-content/uploads/2017/06/GW-BrutoTroll6.jpg

The picture is of a figure but it's the clearest one of his design.

Goblins and orcs like to torture and maim, instead if helping the wounded they often just get them drunk and amputate their body parts like with Azog, or patch them up with metal ( like in the shadows of Mordor games but those are not canon to the movies)

His extremities are cut off, instead of legs there are these unstable mace-like prostethics and chained weapons for arms, if he falls down how would he even get up? Also he is BLIND, there are hooks in his eye sockets which are used by his rider to change directions.

How is this strategically better? Also i don't want answers like " it makes no sense" or " this is a shitty lazily made movie", that might be true, but i still want in-universe answers or at least serious speculations or theories.