the ISM VPN module is an hardware based module for performing encrpytion / de-encryption and its purpose is to reduce usage of main cpu resources in the ISR G2 router.
Licenses are part of licensing model introduced with ISR G2 and provide the right to run some advanced features that would be otherwise disabled. Among these features there are those related to encryption.
SEC-K9 has some limitations in terms of number of tunnels and total encrypted traffic throughput.
So you need the right license installed to be able to configure and run the feature, you may need the HW based ISM VPN module if the aggregate encrypted traffic volume is so high that it would load too much the main processor.
So I would see the ISM VPN as a good companion for the HSEC-K9 license