AngularJS - why manipulating DOM in controller is a bad thing?

Question

Its common knowledge that DOM manipulations should not be performed in AngularJS Controller, its very hard however to find why exactly is it a bad thing. All the sources say

Answer 1

The reason it is more difficult to prove their point with a code sample is that the reason can't really be represented by a short code snippet (short enough for Stack Overflow). It is really a maintainability precaution. Over the long term, you want to be able to independently alter the logic behind controllers and views independently, because otherwise a coupled controller and view pair tend to stay that way and limit each other in their ability to change their functionality without breaking the other. As soon as you decide to change anything about the view, you have the chance of making your controller code break without even touching it.

Testing becomes easier over time because the more tests you have, the more you wish that things were more modular and dependent on as little variables and parameters as possible.

Again, it is maintenance that drives this suggestion. The problems listed above might not be that bad starting out. But imagine adopting a project that you didn't build from the ground up and know all the intricacies behind the coupling between controller and view that hold this application together. What if your application reaches so many thousands of lines of code that it would be impossible for you to know all these intricacies even if you DID build it from the ground up?

For a more general understanding of why design patterns like the one you have alluded to are necessary, you can refer to this google search that will take you on a journey as long as you are willing to take. And for a general understanding of why design patterns even exist and why many people end up suggesting the same thing over and over again, you can refer to one of the catalysts to the introduction of design patterns, Christopher Alexander. He shows us that patterns are what they are because they work well and people repeat what works well.