Why should a manager or leader create challenges for his assistants and assign them difficult and important tasks?

Thanks

I support my colleagues answers

Assigning assistants difficult and important tasks, will create them to think and develop, it can be seen as training on the job, preparing them for a higher position.

If you do this as manager, do keep an eye on it, but in a way that it does not make them feel being watched.

After the task is completed evaluate with them how things went, have within this evaluation a positive approach, remember you are there to build them up.