They are primarily an inspection tool, and can be used to set a sine bar or sine plate. I have a set that I rate only for shop use, since the cal date is out. I stack them for ease of setting a depth stop on the mill, for instance. Since center drills or spotting drills are short, you need more quill travel, but when drilling to a fixed depth, a block makes a handy stop. If also counterboring during the same operation, there is another "fixed" depth. Of course, this is helpful for production work.
The standard use for gage blocks (I don't like calling them jo blocks for some reason) is on the surface plate along with a height gage using a test indicator to verify length of a part in the vertical plane, or verify step to step dimensions. They are useful, with proper accessories, to set bore gages, other gages such as Gagemaker thread measuring instruments, etc..
Of course, I have seen, and done myself on occasion, people use them to provide go/no go gages for splines and keyways. That's not what they are for, but they work, but it is hard on them.