This prompts a more interesting question: How much longer until all of my personal information of this sort becomes accessible to anyone who wants it? What will we lose when it does?
[1] https://theworknumber.com/
Equifax also relies on employers to report data to them, so if your employer doesn't, then the existence of the Work Number is irrelevant. In the era of ubiquitous side gigs where people get paid as contingency workers rather than wages or salaries by the company, you run into issues relying on the Work Number, even before factoring in all the other types of income.
I've been getting regular spam from a credit card company to update my income as well. They probably just want another point of confirmation, or it's just a simple case of the left hand doesn't know what the right is doing.
That being said I suspect this could be a mechanism to "verify" or confirm things are still accurate or current from what you last said it was.
Imagine you telling your CC company you make 250k a year to get a higher limit but only make 100k. Perhaps that credit card account has a large amount of cash flow due to expenses being refunded from an employer or something. A way to find that out could be to ask you to update what you make every so often hoping you'll do the "math" wrong.
If it's a CC company they might also just want to give you a higher credit limit - it's how they make money after all!