if you create a block without background and without margins, its content will stick to edges of the screen (bad)

It's not bad, because

- we have design of block with padding

- padding can be easily inserted manually

- if you really need edge-to-edge design (for image for example) it can be achieved by default

Now with paddings we have a lot of problems:

- lost space on mobile devices

- can't make custom block with edge-to-edge design

- also custom blocks without background and without margins are very rare, while in all other 99% cases space is used ineffectively

Also tendency isn't good, and soon we'll have block inside block inside block inside block inside block inside block... 

In the beginning we had completely borderless blocks, then we introduced block borders, then blocks inside blocks, now we added spacing around blocks to make blocks to look like blocks on mobile devices, I think this tendency isn't good. I think with such approach we are in the middle of this picture somewhere at the end of step 1 🙉