Layout Grids are everything in UI Design.
Well. OK. Don’t discount Colour. Don’t forget about Typography. But get your Layout Grids right, and you're half-way there in creating much better looking, and functioning UIs.
Consistent, and scalable spacing helps you eliminate guesswork whilst designing and developing. It requires fewer design decisions. And it enables a much faster turnaround on projects.
The 8pt Grid is probably one of the most commonly used Layout Grid Systems there is, and for good reason.
PDF included for Free when you Become a Paid Member