Typically, we use them on the internet edge for protecting customer networks from the internet. It's a delimiter between the local area network and the wider internet. Other use cases include securing data centers or protecting certain areas within a network. It's not particularly internet-based, but it gives you that added layer of security between networks or between VLANs and your network, rather than using a Layer 3 switch.
Ultimately, it's about securing data. Data is like your crown jewels and you need to be able to secure it from different user groups. Obviously, you need to protect your data from the internet and that's why we generally deploy Cisco ASAs.