TAD Docs

From version 6.8.8.5 onwards you can even create a new object by dragging a rectangle with the mouse

Note when grid is turned on
If the grid is turned on, the dimension of the created object would be readjusted to be as per a multiple of the current stepsize of the architect; instead of the free form rectangle you created. TAD will try to match, to the best possible extent to ensure that this redimensioned object is on the grid, but please note that such objects created when the grid is turned on; may not be on the grid with relative to other objects on the site.

That means, each object created when the grid is switched on would surely be multiples of the current architects size (i.e. stepsize) but the gaps between objects may not be such multiple. Even then, such a feature is useful when you want to create quite exact size rectangles for your objects – later on then you only have to reposition the objects as per your taste; but you need not resize them

See also: using the "architect" as a spacer