Why to play pure GK list?

The whole making their points back rubbish is a misnomer. Not every thing you take will even come close to it, it's a units effect on your army and your opponents that matters.

I've often dropped a Brother Captain with psycannon behind a tank and got his points back but at the same time, I struggle to kill enough with a land raider to even come close. But that Land Raider has soaked up a lot of fire, protected my troops and forced my opponent to reconsider their actions.

I agree. Model can earn back its points several ways and destroying enemy is only one part of being usefull.