Does anyone know why ERG kidnapped our heroes?

Okay, I think we need to clarify a few issues to better answer this question. When you say "ERG" I am presuming you mean Claire's adoptive father, Mr. Bennett. If this is correct, then it is actually HRG, for Horned Rimmed Glasses man. Secondly, you have to distinguish HRG from The Company. HRG was the main 'avatar' of the Company during the series, but it is actually the Company that is doing the kidnapping. Now that those two issues are brought out, we can work on finding the best answers to this very good question. Now so far the Company has been shown to be tagging 'evolvee' targets. The question then becomes, Why? Why would an organization be interested in tracking and documenting the newly developing 'evolvees' of this world. The answer is relatively simple. Say you were like Linderman, trying to unify the world by first causing or allowing an atrocity to be committed. First off, you would need to know the types of abilities that would allow you to pull it off best. How can you best select these abilities? Start and develop a database of evolvee abilities and keep tabs on their whereabouts and progression over time by using some kind of tracking/tagging system. This would allow you to determine what ability options you have, while at the same time, determining when they are ready to harvest, and what abilities or evolvees you need to keep an eye on that could pose a problem to your very large plan. This is one reason for why they are kidnapping them, but there may be aspects that I could be over looking.

Because he wants to manipulate the power, the power to change the world by liquidating the heroes and held the world at his hands.