They can be handy. One situation where it saved the day - my friend has a partially broken USB-C port on their phone that only charges when the cable is just right. The battery had died so there was no way to see if it was charging or not. Plugged in a USB-C inline power meter and we moved the cable around until the meter showed current, then we carefully set it down so it would finish charging. :)
I've had issues with USB devices drawing more current then the port could put out. Without a power meter, it looks like the device just doesn't work (although the Linux kernel logs typically show a device disconnect event).
With the meter, you can see a current peak up to a nice round value, followed by a voltage drop. The low refresh rate makes it not nearly as clear as with an oscilloscope, but once you know what to look for, it is easy enough to pick up. (And if you really want to, you can get one with a breakout to connect scope probes).
If this is an issue you run into often, I'd also suggest a usb variable load tester to check exactly how much current a port will output.
Suppose you plugged in your Phone/Tablet and it is not charging or charging very slowly etc. Problems might be;
1) The battery is dead and cannot be charged.
2) The cable is a non-charging one or internally damaged/dead or non-fastcharging or with limited fastcharging capability.
3) The charger is not capable enough to supply the max current that the device can use to charge itself.
To troubleshoot the problem, first check the Voltage/Current specs for your device, make sure that the charger plug/battery can provide the same voltage but same or higher amount of current. Then use a USB power meter to see what is actually being drawn (or not) to figure out where and what the problem might be. One key point to note is that for fastcharging all the links in the chain i.e. charger, cable and device should support it.
I have a gaming laptop for which noise and heat are directly correlated with power draw, but not always so obviously correlated with graphics settings, and having a power readout on the cable let me see what different low vs medium shadows had, etc.
One thing you can do is check battery health by observing the charging process with a power meter that supports charge counting.
Another use case was when the battery in my laptop was completely empty, which made it switch to some emergency charging mode that didn't like the normally fine USB-C power supply. I found that it did accept charge from an external battery, so I charged it using that for a while until it booted.
I have a cheap device that costs about 2€ on AliExpress. It is not a cable but a little box.
I don't remember why I got the older one, but the USB-C one was mostly just curiosity - I'd heard that wireless charging was inefficient compared to wired charging and wanted to see how much.
