Mobile IPsec allows creation of a so-called “Road Warrior” style VPN, named after the variable nature of anyone who is not in the office that needs to connect back to the main network. It can be a sales person using Wi-Fi on a business trip, the boss from his limo via 3G modem, or a programmer working from their broadband line at home. Most of these will be forced to deal with dynamic IP addresses, and often will not even know the IP address they have. Without a router or firewall supporting IPsec, a traditional IPsec tunnel will not work. In telecommuting scenarios, it’s usually undesirable and unnecessary to connect the user’s entire home network to the office network, and doing so can introduce routing complications. This is where IPsec Mobile Clients are most useful.
There is only one definition for Mobile IPsec on pfSense, so Instead of relying on a fixed address for the remote end of the tunnel, Mobile IPsec uses some form of authentication to allow a username to be distinguished. This could be a username and password with IKEv2 and EAP or xauth, or a per-user Identifier and Pre-Shared Key pair, or a certificate.