Debuff success increase effects are useful for Servants able to perform directly game-changing debuffs (such as Charm or Stun). However, such Servants are rather rare, and other alternatives are usually preferred.
Useful for Servants with spammable debuffs, especially if said debuffs are Charm or Stun. However, this effect will only be significant for AoE-debuffs, and it would be a waste to use it on Servants with single-target debuffs.