Use of grid in PCB design
As with other vector drawing software the grid exists to assist the operator in getting a tidy layout.  

with some parts dimensioned in inch fractions and other in millimetres. having both metric and inch grids available is useful.

I find myself using several different grid sizes depending on the environment in the part of the board I am designing.

Basically the grid exists to eliminate the need to draw freehand. working around inch parts I use a fractional inch grid, around millimetre parts, a fractional millimetre grid.