Look at the corporate raiders of the 1980s. Firing people, loading up a company with debt. And taking them bankrupt makes a ton of money for private equity. No product required, immensely profitable!
If you're licensing a patent to others with reasonable terms, it's not a patent troll.
I understand you think they provide a service since you seem to not understand what a patent troll is.
As a reminder, a patent troll company is a company "that attempts to enforce patent rights against accused infringers far beyond the patent's actual value or contribution to the prior art" (https://en.wikipedia.org/wiki/Patent_troll)
They don't sell any services, don't do any (reasonable) licensing, nor provide any goods.
Seems like if you've only come up with patent trolls, then "plenty of companies" doesn't apply. Patent trolls weaponise the legal system against other companies. They exist because the government can't make a good enough patent system.
They don't seem to be significant enough to the overall general statement that companies exist to serve their customers with goods and/or services.