A properly designed captive portal can make it harder for someone leaving a small computer in the vicinity of your network and reverse tunneling through it for

A properly designed captive portal can make it harder for someone leaving a small computer in the vicinity of your network and reverse tunneling through it for nefarious purposes. Having to log in every day would disrupt the tunnel and prevent remote access. (Un)fortunately many captive portal are poorly designed, allowing arbitrary traffic to be sent over UDP/53 or ICMP, even when not authenticated. This kind of defeats the purpose of a captive portal, as the tunnel can be operated over UDP/53 (or whatever) and the captive portal can be solved remotely.