Creating a cron job using a while loop

There are several libraries (especially for NodeJS and Javascript) that allow you to implement cron jobs and subsequently host them on a server.

In essence, cron jobs seem to me nothing more than repetitive tasks that are executed at a specific time/date on a day.

I was wondering therefore what the difference is between these libraries and just let's say a custom while loop. For instance in Javascript we could write:

var keepRunning = true
while (keepRunning) {
    setTimeout(function () {
        // call function to be executed when time constraint satisfied
    }, 5000);
}

My questions are therefore:

• why do we use cron job libraries? What are the benefits above a custom function like above?

via JohnAndrews