What is the common application of a Power Resistor Decade Box?
You found a special one (in my opinion) as it is a power resistor decade. Having a look at the linked specification, it is able to dissipate 225 W. That is huge.

So I can imagine this thing being used as a means to test power supplies before electronic loads where used for that. So you can adjust the load of the supply with your decade. Just be careful in which direction you switch you stuff, you can easily create a short circuit if you set it to 10 kOhm and want to test 100 Ohm and turn down the kOhm dial to zero before setting the 100 Ohm dial.

I use the Danbridge decades quite regularly in my work to test the temperature resistor measurement part of our sensors. If you have a precision decade it's much easier that hooking up different resistors. You can easily test the linearity and measurement range for example.

Another use is to test the low power LEDs we use to see the difference in brightness with varying current. In our designs it's often not feasible to implement a constant current design, so we stick to plain voltage and resistor. To get the right value resistor, these boxes are quite useful.

As for your second question if yours is okay:


It looks mostly okay. Have you zeroed your test leads? In the low ohmic region it looks like you might have the test leads resistance being a factor there (mine are often in the region of 0.3 Ohms).
Other then that only 100 kOhm stand out with -3.8 %.

According to your link, it should be within 2 % and 5 % for the 1 Ohm range. Yours is borderline on the 1 and 10 ohms but pretty good in the rest.