Its not really a giveaway per se. Basically Rollbit is buying NFT that is popular on the market and after they bought those NFT, they are probably going to 'resell' it back through NFT marketplace betting or through rollboxes. This is to encourage people to wager on the NFT marketplace betting because there is a slim chance of getting that NFT with much cheaper price
Aside from that, this will attract alot potential customer to try their site so yeah this is basically marketing as well and its a smart one.
The original NFT will has a blue tick on its name