Telemarketing is useful in some way to the person who employs the telemarketer. Otherwise he/she wouldn't pay someone to do it.
It's also useful to the person earning the salary, who then spends their salary consuming in the economy. Not everyone is lucky/brave enough to pursue what they are truly passionate about.
Finally, even if the job doesn't generate any sales, it's still redistributing wealth away from one set of people (shareholders of the company) to others in the economy. Is this useful?
This job is based on lying and/or deceiving to people to trick them into buying somethng. Whoever is paying for it is actively hurting other humans. In a way it's worse than bullshit - it's a malicious job.
And how do you define usefulness for humanity?
Yes, please do. I want to hear what kind of bizarre reality you live in where telemarketing is useful.
I recently received a call to remember to sign up for a charity race before the early signup deadline passed.
It was good for me because I had done it for the past 5 years, but hadn't remembered to sign up yet. The reminder saved me money for early signup.
It was good for the caller because they then had another person committed to doing the event.
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The college I went to was funded in part by donations from alumni. Without donations received by telemarketers calling, there would be less resources available for my education.
Lobbyists are critical to the functioning of the American system of government. They're even mentioned in the Constitution.
Private equity firms make retirement possible.
PR researchers enable companies to not have to deal exclusively with the populist swaying of public opinion. Public opinion, generally speaking, is an awful way to do business and to make decisions. Mobs are cruel.
I could go on but this method of dealing with your issues is unsustainable. Suffice it to say that you're just going to have to live with the fact that you don't understand everything and that's okay.