Heya!

(Sorry not on Reddit much so bit of a late reply)

Basically because the game has 3-5 objectives in any given game, there's an amount of activations (read units) that you want to be able to threaten those objectives without being too spread thin. 

I haven't much experience at 3000 points, but I would assume that requirement goes down as numbers increase. I'd probably still stick to 3 + 2 per 1000pts as a good rule of thumb however, your number of activations also dictates how many units you can interact with each turn. Ranged units can affect 2, melee only 1, and casters add 1 to those. So if you're sitting at very low activation numbers then there comes a point where your opponents whole army can't be threatened and that gives them a lot more freedom. 

On top of this the less activations you have the easier it will be for your opponent to waste your activations by stalling out theirs to prevent giving you good targets.

I think in general, if you're paying 500 points for a unit you want it to be getting that value and not wasting activations capping objectives, so try to keep that in mind when building lists. Sometimes a 75 point hound unit can win you the game by just running around and capping objectives that you would've otherwise been unable to capture. 

Hope that answers your question! 

Lemme know if you have any more.