These blocks are generally used for set-up. For example, they are useful for setting a work piece up off the table so that you can drill thru it. You can also screw two together to make an angle plate (as long as they have the corresponding threaded and through hole), they're square and can be used as a square.
You can secure your work with a clamping kit directly into the threaded holes. You can use your clamps to pass through the block, to secure your work, and then toss it in a vise for odd shapes that would normally not line up with t slots. You can secure your work with the threaded holes, and then pass though the unthreaded holes to secure your work and block to the t slots for milling. This also applies to 2x4x6 blocks.