THEN (and this is the part you don't seem to understand) the client process has to waste time solving the challenge, which is, by the way, orders of magnitudes lighter on the server than serving the actual meaningful content, or cancel the request. If a new request is sent during that time, it will still have to waste time solving the challenge. The scraper will get through eventually, but the challenge delays the response and reduces the load on the server because while the scrapers are busy computing, it doesn't have to serve meaningful content to them.