These can be valuable to you in several ways. First, if one server goes down, another is there to keep the VPN alive.
Some vendors even keep alive the individual tunnels between the downed server and the remote user.
Second, if the devices can share loads, performance won't degrade during periods of heavy use. Doubling up VPN devices at key sites may sound pretty expensive, but redundancy at each site is not necessarily a requirement to protect a VPN from a server failure. For example, some vendors have client software that will automatically seek a server at another corporate site when it detects a server failure at the other end of a VPN tunnel. There is no need for two servers at a single site. Failover between sites requires the remote client to set up a new tunnel, but that can be transparent to the user.