Aesthetics,ambience.
Usually you fit downlighters no more than 600mm way from a wall, this is width of a kitchen unit,wardrobe.
And distance between fittings 800-1200mm, it all depends on the lighting you wish to achieve, & the beam spread of the lamp, narrow spot or wide flood, etc, & wattage, this will vary due to height of room, but you want the beam spread to overlap slightly, so you don't have define areas of light, but it merges in to uniform lighting of the area.
You might get away with just two rows of downlighters, with three or four fittings to a row for a bedroom, whereas a lounge might need three rows, to achieve the desired lighting level, all depends on size of room as well, it then looks balanced, even numbers better than odd numbers.
Also I prefer each row independently switched/ ideally controlled with a dimmer, gives you more choice of lighting depending on your mood,needs.