This HSEC license is needed to provide greater VPN throughput and tunnel counts over the standard SEC license. This has to be purchased in addition to the SEC license, or purchased above the 29XX-SEC router bundle. Interestingly enough, this HSEC license is included with the ISM module, so if you think you need the benefit of the ISM offloading, then it does lessen the overal cost a bit.
The SEC license allow max VPN throughput of 85Mb and 225 tunnels. HSEC is "export restricted" license, as it was explained to me, and will open the SEC licenses restrictions to the router's full capacity. We ran into a tunnel count issue and had to learn this the hard way.
If you are in a scenerio where 85Mb (per direction) and 225 max tunnel count is functional, then there would be no reason to purchase the HSEC license.
The Configuration Guide explains it best.
The HSECK9 license is required for a feature to have full crypto functionality. Without the HSECK9 license, only 225 secure tunnels and 85 Mbps of crypto bandwidth would be available. The HSECK9 license allows features in the securityk9 technology package to use the maximum number of secure tunnels and crypto bandwidth. To enable the HSECK9 license, purchase the FL-44-HSEC-K9 license from Cisco.com and install it using the license install license-files command. For further information on obtaining and installing feature licenses, see Configuring the Cisco IOS Software Activation Feature.
The HSECK9 feature does not have an evaluation license that converts to an RTU license after 60 days; a feature license must be obtained.
To enable the license for the HSECK9 feature, the securityk9 technology package is also required. For more information about thesecurityk9 technology package, see securityk9.