Each grid size in Dromed simply doubles the snap size of all brushes, plain and simple. Here's a chart:

Grid size Placement Brush sizes

11 0.125units 0.25units
12 0.25units 0.5units
13 0.5units 1unit
14 1unit 2units
15 2units 4units
16 4units 8units

So at grid size 14 you can make brushes in sizes of multiples of 2units (such as a brush that is 12 wide, 10 tall, and 8 long) and you can place that brush in 1unit increments, such as moving the whole thing over 1 unit or 4 units or 7 units (but not 7.5units). That's why you don't need to go down to 11 to make your stairs .75 tall. Yeah, only on 11 can you make the actual brushs .75 tall, but in grid size 12 you can position 1unit steps to slightly overlap and make the visible steps remain .75 tall, because you can place brushes in .25 increments at that grid size.

The best reason to use larger grid sizes early on, in my opinion, is texturing. Textures always fit best on walls that are sized in multiples of 4 or 8, but also textures will mostly be all pre-alighned already if you built at a large gridsize so that your initial placement of the brushes is also in coordinate settings of multiples of 4 or 8. Otherwise you'll find yourself having to do a TON of U and V adjusting later on. And even if some textures still need to be aligned later on, if you've started at a high grid size and have your construction largely at nice coordinate then you can enter into the U and V boxes nice round numbers like 32 or 64 and you're likely to get perfect alignment right away instead of having to cycle through all the numbers to see what "looks good".