The following scenario should illustrate the purpose of a secondary address:
Your network is using an ip addressing scheme of 15.0.0.0/24. Subnet 15.0.1.0 has had all 254 host addresses allocated, yet you now require more addresses in this subnet. What do you do? Assuming that subnet 15.0.2.0 has already been allocated, you are quite limited in what you can do. Enlarging the subnet by changing the subnet mask will result in an overlap with 15.0.2.0, so that won't work. You could change the subnet mask of the whole network to give you more hosts, but that would require you to change the ip config of each existing host. The simplest solution is to add a secondary address to the interface of the subnet in question, thereby adding a second subnet with another 254 hosts. Let's say you add subnet 15.0.3.0. So, you now have two subnets hanging off the one interface, 15.0.1.0 and 15.0.3.0, with the router forwarding packets between the two. I hope that makes sense.
Cheers.
