What are the benefits of a dual SIM card?

See the following from Using Dual SIM with an eSIM - Apple Support

Here are a few of the many ways you can use Dual SIM:

• Use one number for business and another number for personal calls.
• Add a local data plan when you travel outside of the country or region.
• Have separate voice and data plans.

Both of your phone numbers can make and receive voice calls and send and receive SMS and MMS.2 Your iPhone can use one cellular data network at a time.

Read here: Using Dual SIM with an eSIM - Apple Support