Bad Behavior prevents spammers from ever delivering their junk, and in many cases, from ever reading your site in the first place.
Most of the time, only spammers see this. In the rare event a human winds up here, a way out is provided. This may involve removing malicious software from the user's computer, changing firewall settings or other simple fixes which will immediately grant access again.
Bad Behavior's built in log viewer (WordPress) shows why requests were blocked and allows you to click on any IP address, user-agent string or block reason to filter results.