They are vital in the same way mosquitoes are vital to the ecosystem. You try to assume they must serve some higher purpose greater than blood-sucking(eg food for fish), but nope that's it.
I fail to understand how I assume they serve a greater purpose -- they simply provide consequences for debtors, something increasingly difficult. I hate them, but it fascinates me that some folks can't even bear to discuss how the meat grinder actually works.
Because grind it does.
You stated they are vital. This is an assumption. They are not vital. The real consequences are already part of the system and the collection agencies serve no purpose but to harass and annoy.