• It scales perfectly in all the different screen displays (including android’s @0.75 and @1.5).
• Apple and Google advise so.
• It's a good basic unit to work with. 4 and 8 are easily multiplied, are flexible and consistent. 

We use small artboards while the devices we design for are huge. For example:

Icons designed on an 8pt based grid will scale perfectly.

... using the extended framework for the 8pt grid: Typography, Icons and Layout.

The standard Bootstrap grid is a 12 column layout, with a 15px margin on each side of the column, resulting in a 30px gutter between columns plus 15px to the left and right of the grid. 

While the font sizes can vary and move away from the 8 multiplication, the line height will not.