for each position, what does great athleticism rating mean for your players? how does having better athleticism for a player effect your game planning decisions?

Well. It seems to lead to increased free throws with me, all else being equal. . .

Athleticism is manna from heaven.

It helps your shooting (low post and perimeter), helps your defense, helps your rebounding. Imagine athleticism to mean vertical jump and lateral quickness. Even if your player is fast, if he is struggles to change direction, or get up high, than he is going to have nights where he gets crushed.

It also messures strength. Explosiveness and finishing drives and getting to the foul line more.

I think athleticism is very important for your scorers no matter the position. For example in D3 an 18ath/85sp pg who you only rely on to be a distributor will do very well depending on his bh/pa. But I'd take 60ath/60sp SG with the same cores as 18ath/85sp SG. I'm sure some will disagree with me on this but the huge advantage in ath outweighs the speed advantage IMO.