Nitan
The answer about why to use the secondary address option in HSRP is that you may want to provide the same functionality (having the end stations gateway address able to failover from one router to another) for end stations whose addresses are in the secondary subnet.
For example you may have a pair of routers doing HSRP using shared address 172.19.22.1 for end stations in subnet 172.19.22.0/28. So the end stations configure their default gateway to be 172.19.22.1 and they can get to remote resources even if the primary router fails. So what would be the case if that interface had secondary addresses configured for subnet 172.19.24.128/28. If you want those end stations to have a reliable gateway then you need a shared address in that subnet also. The secondary capability of HSRP is how you do that.
So I would say that any time that you have interfaces configured with secondary addresses and are configuring HSRP for the primary address, you probably want to also have HSRP for the secondary address.
HTH
Rick
HTH
Rick
