I guess, the 12" MacBook was designed on a promise of Intel of delivering processors, they never could deliver due to the issues with their 10nm process. This is where an ARM based design around the iPad processor would be an instant win performance wise.
But it is also great news, if Apple really uses the costs saved by going with their own CPUs to offer the machine at a lower price. Suddenly, it can compete with a lot of branded laptops on the price alone. With the additional speed from the processors, it should be quite a good general purpose laptop. And we know how nice the 12" MacBook was from a design perspective.
For the consumer market, the ability to run iOS apps becomes an additional huge benefit. For more and more people, the smartphone is the default computing platform, especially for younger people, the smartphone might be the device they get into computing at all. So when bying a laptop to go along with your smartphone, the Mac running Apple Silicon has a huge benefit, as it integrates more seamless than ever with your smartphone.
I would expect the Mac market share to get quite a boost as a consequence.
While the MacBook is the obvious machine to equip with Apple Silicon for the reasons listed above, I would also expect Apple to hit the market with a true developer machine just so that developers can not only test their software on Apple Silicon, but that it becomes the development platform right from the start.