why would a person or company set up a site to site vpn emanating from an Cisco ASA 5515 rather than from a Cisco 2951
I"m asking about Cisco because that's what I have. But you can use more generic examples if that's what your comfortable with.
I'm just trying to understand the rationale behind using the firewall rathe than the router.
The box you use for VPN needs to have VPN firmware and capability built into it. If the Firewall box has this capability, there is no reason not to use it.
Generally I see VPN built into routers. So I have a Cisco RV042G VPN router in my home office and it has site to site tunnels with clients who have Juniper Netscreen boxes. These are essentially routers as well.
So to directly answer your question, if the Firewall in question has VPN capability, then no reason not use it.
A routers primary job is Routing, companies add more feature to it, but that's its primary function.
A firewall appliance like ASA's primary job is to protect trusted traffic from untrusted. Allowing only required traffic to pass. It has hardware resource to smartly process the encryption/hashing computations. It does content filtering, IPS/IDS and manymore, which for a router is too much to do.
Cisco ASA 5515 is a hardware firewall appliance, wherein Cisco 2951 is a router. Site-to-site and Remote Access VPN is primarily ASA's job than 2951 router.
Miftaul is correct, the ASA is designed as a firewall/VPN device. Routers are for routing and connecting varying media (Ethernet and Frame relay for example).
If you have both a router and an Cisco 2901-SEC ASA, it makes sense to terminate the VPNs (site to site and remote access) in the ASA.