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.
#Connect in to AAISP over a third-party internet connection. This gives you your an AAISP IPv4 address and an IPv6 block. There are more reasons and information on http://aa.net.uk/broadband-l2tp.html