DHT crawlers/indexers already exist to perform that function; they crawl and store infohashes (+ metadata when they receive it) and allow users to search that metadata to return relevant infohashes
By announcing itself, the indexer makes itself more likely to be handed out as a peer to anyone else interested in that infohash. Every connection attempt it subsequently receives is evidence of another peer announcing or joining that torrent. In effect, it "baits" peers into revealing themselves