Are all these electronic loads just for stressing power supplies?
That's their main function, but it's not the only one.
I have a feeling a lot of us would be more interested in measuring performance for battery operated devices, using lipo or supercap.
Notice that this is a load that will do such things too. Please look at some of Dave's videos regarding battery performance, and you'll see that it's very useful for just that.
Looking at standby power, sleep modes, etc.
No, that's the other way around. For this you would want to use a lab supply and/or multimeter with datalogging capabilities.
Im suprised that the keithley looks to be rebadged, is that true? Is similar firmware, with new face plate?
I'm not, lower end stuff by Keysight, Tektronix, LeCroy, etc. are often rebrands nowadays.
Other wise what options are available for monitoring low power devices as I mentioned above?
An electronic device can be separated in the part that does what it's meant to do (the application) and its power source. The latter can be a battery, a solar cell, a PSU, to name a few.
A load is used to find out about the characteristcs of a power source. Looking at what the application circuit draws, etc. you're looking at voltmeters, currentmeters, power meters, oscilloscopes...
The other extreme seems to be the ebay load kits etc. But I suspect the software would be more important to recording performance etc.
A simple DC load is easy to make. Just a power resistor or a bulb can do that job, but only that job and only at a single load. A DC-load using active devices can be manipulated so that it can funtion over a wider range, but a DC load like this BKP 86xx is essentially microprocessor controlled. That way it's more versatile and you can e.g. simulate lots of different loads, e.g. a pulsed load to simulate an actual device switching in and out, and log the data in software.