Primary advantage of DirectX12:
A Directx12 game would utilize your CPU a lot more efficiently. Directx11 only utilizes one of your CPU cores for certain tasks. Only one! And my high end Intel Core i7-6700k 4.2Ghz CPU has 4 cores and 8 threads. My CPU is not even breaking a sweat playing this game. Of course all of my cores and threads are being used, but..... they are not being used to my CPU's full potential.
Since this game is simulation based, simulation games require a good CPU. But since it's Directx11 from Windows 10 Task Manager and MSI Afterburner results, when playing this game, my CPU is not being utilized to it's full potential.
Now when I edit my YouTube videos using Power Director 16, that program utilizes my CPU more efficiently than this non-Directx12 game. If Frontier (and that's a big if) ported this game to Directx12, we would for sure see some frame rate improvement for this game. Not talking like 240fps, but it would be like a 10fps or a bit more, or at least some noticeable improvement in game performance.
An issue is that the majority people don't have Windows 10. The majority of people do use a Windows OS though, but many of them are on Windows 7 or 8. There are reasons why people don't like 10. Data harvesting, privacy issues or concerns especially around Cortana collecting your data. And 10 being referred to a Service than an OS like all previous Windows versions.
But it sounds great if this news of them putting it with Windows 7. More people still use Directx11 systems which is why Frontier made the game for Directx11. It's why people who have systems with a good CPU complain why this game runs into poorer fps. Otherwise it's a great game to play. Note if you add too much to this game, like any other simulation game, it will lag eventually, regardless if it's x11 or x12. You have to do quite a lot to make The Sims 4 lag.