Well, dual SIMs is a useful feature if you have a work and home number and want to get calls on one phone. Of if you travel a lot and want a local sim.
The trick here is that starting with the iPhone XS, Apple supports an eSIM, that is an electronic SIM, so no card required, and a physical sim.
So instead of activating with a carrier and needing a sim, you just all them up, read out the eSIM number and you are connected.
Then, when you want the second line from work or for travel, you can insert a physical SIM and suddenly one phone has two numbers.
Once you have this all set up, then you click on Turn on this Line and you can actually label the two SIMs (work, home or France, Canada, etc). You can set a default line as well.
Now in contacts, you can label contacts with the Preferred Cellular Plan to use to contact them. Since you can also set a default this is really convenient. Also, if your home carrier supports Wi-Fi calling, then you can answer calls from it automatically.
And when you make a call, you can press in the Info button and change the number you are using. Same with iMessage.
There are also going to be two lines on the upper left that show the status of the two lines which is really handy and which is using WiF and which is using cellular data.
And amazingly, they made all this work with the Apple Watch too