Using Single Responsibility Principle

When i start writing a new piece of code, why do i want an arbitrary separation of all the bits that are changing together? Because they're in my head right now. If you ever did any enterprise jave back in the day, most of your life was copying and pasting cod between getters and setters. So it's, again, i describe it as neither necessary nor sufficient if you do it. I mean, maybe the other thing, ill ad i'll hut up because i levr got a lot more werinons.