Why Use a Modular Grid in iOS App Design?

Question

iOS

Actually, I saw such a thing

. Question: why do I need this when designing and how to use it?
Follow-up question: what grid spacing (in FS) should I use on @1x? In 1 pixel?
Follow-up question 2: if I draw at @1x, how do I prepare bitmap graphics? background, for example. Do I need to cut this background for each resolution (iphone5, 6, 6+) ?
Thanks in advance.

Answer 1

You make graphics under 6+ in a resolution of 1242 by 2208 from it you are already resizing. @1x is only used on iPads.