Similar to your modem case, the fibre ONT on the side of my house is now PoE powered, because it would otherwise need two pairs from the CAT6 cable to provide 12v to itself, from a backup battery supply inside the house. Replacing that supply with PoE, this allowed me to centralize my network stack’s power source, so that a single UPS in my networking closet can power that ONT. It also reflects the reality that if my PoE switch goes down, my network is hosed anyway. There was also the issue that with only two remaining pairs, it would be impossible to realize 1 Gbps on the CAT6.
I also have PoE to the RPi1 units which attach to my TVs. These serve as set-top boxes with interactivity with CEC via the TV’s HDMI port, and are PoE because I insist on all my devices being wired rather than on WiFi, so might as well provide power as well. These use a microUSB PoE splitter, because 1) the RPi PoE hats mean I can’t fit into standard RPi cases, and 2) the PoE hat runs very hot and makes a high frequency squeal, which was unacceptable in this application.
Power cycling via SNMP on the switch is another nice benefit to having stuff PoE powered. In fact, I have one more application which depends on this behavior. I have a blade server which sits in my garage, that would otherwise consume a lot of standby power when I don’t need it. To fix that, a 240vac relay with 12vdc control coil sits ahead of it, so activating the relay turns on the blade server. That relay is powered by PoE, commanded by the switch, so whenever I want the blade server, it’s only an SNMP command away. iDRAC then communicates over the network using that same CAT6 that’s powering the relay, again recognizing the dependency that if PoE fails, the blade server is down anyway.
I’m only using 802.3at power levels right now, as that’s all my switch can do. If I ever acquire an 802.3bt switch, I might consider PoE lighting or PoE phone chargers, or silly things like that. There’s a lot that can be done with 60ish Watts. Note that the efficiency of PoE switches tend to be abysmal when lightly loaded.