Wither rose farming is the process of trapping withers and using them to kill mobs and make them drop wither roses.
Only wither skeletons can spawn in wither roses, so that makes it ideal for wither skeleton farms. They do damage to entities but not items, so it can be used in other farms as a killing mechanism.
JC Playz's design (Bedrock 1.17)
This design is both a wither killer and wither rose farm.