One reason for this requirement is that you want one SIM for your work number and one for your private number, so that you don't have to carry two phones. (You could forward one to the other as a workaround, but that doesn't help with SMS or outgoing calls.)
The second most common reason is if you're split between two countries, home somewhere and home/work abroad, you need to have a SIM for each country.
That's exactly the use case for me. I own a business and don't want to give out my personal number for anything related to the business side. Helps me stay organized. I know when a call is coming through I know the nature of the call before I pick up.
If you use Whatsapp you could have two different whatsapp accounts - one for your business with Whatsapp Business and another regular account. For this you need two SIMs
If you stay outside of your home country but still need to operate, say, bank accounts that require a number from the home country itself, dual SIMs are needed, especially for receiving OTPs etc.
My use case would be I have my regular SIM and I have a SIM/SIMs for international travel. In practice, I use an older phone for this purpose but if I had dual-SIMs I wouldn't need to.