Super useful if you're an expat or travel between two countries often.

Yeah personal and business seems to be the best.

Exactly the options you mentioned, + if you want to have a backup provider in case your primary is struggling somewhere.

I alternate between a Google Fi sim, and an AT&T business.

Those are two, also having different networks for different areas (if you have a good service on carrier 1 at one location and carrier 2 at another)

Yes, both of those.

In some locales, you want to do all of your internet use over one unlimited or high usage internet plan and handle SMS/calls over another carrier or plan. This is appealing for business users and regular people alike in some places that have high tariffs, highly asymmetrical usage limits, or other uneven plan limits that don't align with how you use your mobile device.

For example, I once had a lot of automated alerts sent by SMS to my phone for the early rollout of an IoT device I was working on. My regular unlimited voice and data plan was very cheap but only included a couple hundred SMS per month and I was going to blow past that during alpha testing, so I got a SIM from an MVNO with unlimited messaging for ~$5/mo and sent all the alerts to that number and had the company pay that bill, which was a nice way of isolating the expense as well.

It's also great if you are an international jewel thief and need a second line for taunting the Interpol agents hot on your trail without having to add them to your friends & family plan.

I use one sim card for day-to-day life in the US, and another sim card from my home country to interact with digital services there, when needed.

I currently have my personal and company SIMs. Data goes through the company one, calls through my own.

People who travel a lot usually have their "native" sim as the eSIM and just grab whatever prepaid they can get for the country they're travelling in. Easiest way to avoid insane roaming fees.

I do dual country. One for the US and one for Canada.

Work number and personal number on the same device. I’ve friends that switched their number completely to their work number because of the lack of dual sim phones back in the day. That is just a poor decision all around. I know someone who had to negotiate a release of the phone number to him in the compensation package when they were let go

Data privacy. one esim I use for all these websites requiring a phone number as 2FA or when it's unclear to me why they need a phone number. It's off all the time, unless I expect an SMS on it. It reduce how much my real phone number spreads and hopefully reduces the amount of phishing I get...

I have one with almost unlimited data plan, other for my phone calls/texts.

Different countries is an obvious one. I live in both Spain and the UK.

I have 4 SIM cards due to living in different countries. Sadly only 2 can be active at all times.

It also a diversification for telco shenanigans. Just to give an example, Singapore regulator just ordered telcos to stop accepting incoming SMS from entities not registered with local regulator. This affected a lot of overseas services OTP.

Some people needing a sound alert because they have time sensitive duties. For example, a daycare/school needs to reach a parent.