I guess it is how you look at it. Like you mentioned an RPi can be used for many things on a local network not necessarily related to HE.
Some HE Companion Ideas:
- Offload Rules/Apps for reduced HE overhead using systems like Node-RED.
- Separate UI / Visual for alternate rule apps like WebCoRE.
- External Logging Server.
- External apps that work in conjunction with HE apps to provide missing functionality or connectivity like HubConnect [server], 3rd party dashboard apps, email notifications etc.
Tangentially related stuff that benefits a companion server (and the lan as well):
- MQTT
- NTP Server
- VPN