This way of bidding is usefull to get better controll of bidding. If the opener bids the 3rd suit, this is forcing and can be up to 20HCP. Old way was to jump or reversebid to force, but with 3rd suit forcing the opener gets better "space" to show his hand. If the opener bids a 3rd suit, the responder may only pass with 4-6 HCP if he can see that game against 20 HCP is doubtfull.

2C    : forcing. 12-20 HCP
3C    : at list 5-5 and GF.