Block discs are designed to permanently lock your Limited slip differential (LSD) to 100%.
If you prefer the feeling of a welded differential over LSD, or you would like to try it out, you install block disc set and you have it. Perhaps some drift layouts requires welded differential, so with this set, you can configure your diff to 100% lock all the time. Block discs are widely used on Dragster cars as well as drift cars
What are the advantages of Block discs over Welding the diff?
They are much more reliable. Unlike welded diff, they won't ever crack. You have a Lifetime Warranty.
You can switch back to LSD anytime. Just take the discs out and return clutch plates
Maintenance free. They don't require any special oil or service.
They give you the ability to have one and the same differential for street and drift days. You don't want to wear out your differential on drift days, you can install Block discs and take them out after the race day to switch back to LSD again.