There are two reasons to use L2TP to connect in to AAISP:
#As a backup for your existing AAISP circuit in the event of a fault.
#*Logging in over L2TP using your DSL credentials (ie email@example.com) from a different ISP will then give you your normal IP addresses etc whilst your AAISP connection is being fixed.
#Connect in to AAISP over a third-party internet connection.
#*This gives you your an AAISP IPv4 address and an IPv6 block. This will then give you unfiltered internet access with public IP addresses etc. There are more reasons for doing this and further information on http://aa.net.uk/broadband-l2tp.html