update nullifier-program description with use case context

Author: tilo-14

compressed-pdas/program-examples.mdx

@@ -48,7 +48,7 @@ Full compressed account lifecycle (create, increment, decrement, reset, close):
 
 ## Nullifier Program
 
-* [**nullifier-program**](https://github.com/Lightprotocol/nullifier-program) - Creates a rent-free PDA derived from an id. If the id has been used before, the PDA already exists, causing the instruction to fail.
+* [**nullifier-program**](https://github.com/Lightprotocol/nullifier-program) - For some use cases, such as sending payments, you might want to prevent your onchain instruction from being executed more than once. Creates a rent-free PDA derived from an id. If the id has been used before, the PDA already exists, causing the instruction to fail.
 
 ## ZK-ID Program
 

snippets/overview-tables/program-examples-table.mdx

@@ -8,5 +8,5 @@
 | [Create-and-Update](https://github.com/Lightprotocol/program-examples/tree/main/create-and-update) | Create new compressed accounts and update existing ones within a single instruction and one validity proof |
 | [Read-Only](https://github.com/Lightprotocol/program-examples/tree/main/read-only) | Create compressed accounts and read them on-chain |
 | [Account Comparison](https://github.com/Lightprotocol/program-examples/tree/main/account-comparison) | Compare compressed accounts with standard Solana accounts |
-| [Nullifier Program](https://github.com/Lightprotocol/nullifier-program) | Creates a rent-free PDA derived from an id. If the id has been used before, the PDA already exists, causing the instruction to fail |
+| [Nullifier Program](https://github.com/Lightprotocol/nullifier-program) | For some use cases, such as sending payments, you might want to prevent your onchain instruction from being executed more than once. Creates a rent-free PDA derived from an id. If the id has been used before, the PDA already exists, causing the instruction to fail |
 | [ZK-ID](https://github.com/Lightprotocol/program-examples/tree/main/zk-id) | Program that uses zero-knowledge proofs for identity verification with compressed accounts |