At some point in early HL2 history they were used to remove ragdolls. The current version of Eli hasn't said anything about Cremators apart from commenting on the head - the person that did a slideshow about the Seven Hour War was Eli Maxwell.
It seems as if the Combine might have had a whole area planned to put the Citadel in. Maybe the were working underground on it and they decided to put it in a densely populated area so it can destroy most of the population. It would be nice if Valve made an overhead map of City 17 wouldn't it? That way we can see if the Citadel was tactically planned to teleport to that locale.