Hello,
1. This command defines the IP address that this VPN endpoint uses to identify itself to remote peers. ie the IP on loopback0 is the IP you define in the remote VPN endpoints crypto map statement.
2. You use another sequence number to define another set of VPN parameters. This is needed when you are VPNing to multiple endpoints. ie
crypto map VPN_2_LONDON 10 ipsec-isakmp
set peer 1.1.1.1
set transform-set AES_SHA
match address 101
crypto map VPN_2_PARIS 20 ipsec-isakmp
set peer 2.2.2.2
set transform-set AES_SHA
match address 102
Thanks