Human computer interaction (HCI) is concerned with how humans and other users interact with computers.
This often takes the form of developing user interfaces that are easy for different people to use.
It is relevant for the majority of computer systems (even artificial intelligence systems need to present their information appropriately for end users).
You need to consider HCI for any system that has users interact with it in any way.
The type of interface will depend on these end users. For example expert users should have ways of using the system more efficiently while novices will need something that is easier to use.
Even websites need different user interfaces depending on if they are accessed via touchscreen or a traditional mouse and keyboard interface.