Requiring that people access your site over TCP/IP is a reasonably good way to weed out clients that don't involve computers at all.
Bots are user agents that don't have a human operator; browsers are user agents with a human operator. Both bots and browsers involve computers. However, as browsers can automate some tasks, and bots need human interaction at least to program them in the first place, there is a blurry line between bot and browser. CAPTCHAs try to force a certain amount of human interaction. The only reliable way to prevent human interaction between sending a request to and receiving a response from an unknown user agent is to require a response so fast as to preclude human intervention.
Bots are user agents that don't have a human operator; browsers are user agents with a human operator. Both bots and browsers involve computers. However, as browsers can automate some tasks, and bots need human interaction at least to program them in the first place, there is a blurry line between bot and browser. CAPTCHAs try to force a certain amount of human interaction. The only reliable way to prevent human interaction between sending a request to and receiving a response from an unknown user agent is to require a response so fast as to preclude human intervention.