First, sometimes they should decide cost, human resource, product and so on. And, they communicate with various people and they solve the issues. During the process they get to develop that skills as a leader.
Second, they have rights. They can spend resource like human and finance to achieve objectives.
It makes them getting know how to succeed objectives limited resouces. So they able to manage other things.

Initially, decisions have to be made on things like cost, human resources, product, and so forth. They fix problems by talking to other people. They get to hone such leadership abilities in the process.
They have rights, too. To accomplish their goals, they can invest financial and human resources.
It teaches them how to be successful with limited resouces. As a result, they can handle other tasks.