thank for, reward for
勞 centers on expressing gratitude and giving reward for effort or service: thanking someone and compensating them for their work.
勞 is the traditional form of 労. It combines 力 (strength/effort) with a complex upper element that historically represented exertion or toil, together conveying the idea of labor that deserves thanks or reward.
The bottom 力 shows effort, and the top looks like a person working hard under a roof with fire. Imagine someone toiling away, and you want to thank and reward them for their hard work.
For ロウ, picture a row of workers being thanked and rewarded: row -> ロウ, and you are giving them a reward for their labor.